Best engine oil for the RX8 231?

[ChrisHolmes]

There is no "Best engine Oil for the RX-8 231" if an oil meets the specific requirement of the manufacturer then in theory that is the best but then many variables crop up such as brand, grade, servicing regime, vehicle use etc etc.

Again in theory Mazda arrived at a recommended oil in conjunction with Total Oils which was a 5W-30 grade which over time it was decided by various comparisons with the RX7 which used a 10W-40 grade that it was the grade that was best for the RX-8 to eliminate wear that the RX-8 suffered from but the RX7 didn`t.

So basically if you are searching for the "Best Oil" you will never get a definitive answer due to the many different factors involved eg. I have owned my present RX-8 for 12 years from new and for the majority of that time I have used Shell Helix 10W-40 as I consider it in my case to be the best oil others will disagree and have their own recommendation.

There are recommendations in the Oil 101 section use those and you can`t go far wrong (in theory)

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Del.

[Hopkins]

Don't overthink it

Where's the fun in that

[ChrisHolmes]

Just went trough the majority off this thread looking for a simple answer, turns out there isn't. The internet is littered with different opinions concerning oil for our cars and even shops can't seem to agree on what type of oil to use. What baffles me most is that the three UK shops that pop up when using google or doing research state three different things.

Shop 1: 15w40 (mineral)
Shop 2: 10w40 (SS)
Shop 3: 10w40 (SS) unless you upgrade the oil pressure regulator, you can go back to 5w30...An upgraded oil pressure regulator will only “allow” a higher maximum pressure at higher revs, at idle and low revs it has no effect at all and this is where the 5w30 oil pressure can be very low hence the preference of 10w40 oil. Edit by Chris.

Like some have stated, I agree 5w30 is too thing and was done to meet emissions (while not always beneficial for the engine, lots of other brands have done the same thing).

To my understanding, mineral oil will burn cleaner and leave less residue but will need more changes (in theory).

Can I therefore summerize that when it comes to mineral vs SS it's about reducing carbon build-up vs increasing bearing longevity?

[R1DSO]

Just went trough the majority off this thread looking for a simple answer, turns out there isn't. The internet is littered with different opinions concerning oil for our cars and even shops can't seem to agree on what type of oil to use. What baffles me most is that the three UK shops that pop up when using google or doing research state three different things.

Shop 1: 15w40 (mineral)
Shop 2: 10w40 (SS)
Shop 3: 10w40 (SS) unless you upgrade the oil pressure regulator, you can go back to 5w30...

Like some have stated, I agree 5w30 is too thing and was done to meet emissions (while not always beneficial for the engine, lots of other brands have done the same thing).

To my understanding, mineral oil will burn cleaner and leave less residue but will need more changes (in theory).

Can I therefore summerize that when it comes to mineral vs SS it's about reducing carbon build-up vs increasing bearing longevity?

I went down an oily rabbit hole when I first got my RX-8. Some thoughts that I had when I surfaced for air...

I found it strange that the universal consensus is to go for oil that’s more viscous when hot, when the UK-spec twin oil coolers do such a good job at keeping the oil cool, it’s advisable to cover one up during the winter . I could be mistaken, but this makes me think that an RX-8 running 10w40 on an average UK day would be running less effectively in oil that’s too thick for the temperature.

My car appears to have been run on Mazda’s Dexelia Ultra 5w30 to date (which seems to be less of an issue for R3s and their extra rotor injectors), with the all-important regular changes. The engine performs well and has good compression.

From the first oil change in my ownership, I plan to go for Idemitsu Rotary Racing 10w30. I know comparatively little about oil, but I thought it looked like the best choice as you get the benefits of a fully synthetic oil, but without the deposits - it’s been designed to burn cleanly in rotary engines.

The caveat is that it’s expensive as you can only buy it by the litre, especially if you do a proper double fill and flush to drain the oil out of the coolers. I also add 150ml of Idemitsu premix to every tank of 99 RON, just to be safe .

That’s just my partially subjective 2 cents. There are many more informed people regarding this subject on this forum who may say otherwise .

[ChrisHolmes]

Save your money and follow the majority that recommend using a good prominent brand 10w40 semi synthetic oil.

[v-rex]

Don't think I saw it mentioned but there is one called Synionic Rotary which is a 10W30. I'd never heard it till TPS mentioned it, but looks like it's designed for rotaries. Not sure if anyone uses it and has feedback but sounds good.

I'll be using Millers Trident 10W40 for the moment and first oil change purely as it seems to be used widely with zero problems. Then I'll see about upgrading to one of these fancy ones. Just a reminder that Millers Trident has got a quite good discount at Demon Tweeks at the mo, if anyone wants to top up.

[ChrisHolmes]

Recommended oils are anything depending on which Rotary engine builder, which Rotary OC forum you read, which country you live in, which climate and the age of your engine with grades of 5W-20 to 20W-50 and anything in between.

Even mineral oils versus full synthetics is arguable - Mazda`s stance on that was in a nut shell that full synthetics were never tested in a Rotary so don`t use it then you get into the realms of what exactly is a Mineral oil, which is a semi synthetic and which is a full synthetic, oils can be marketed as synthetic technology which coincidentally is now Total Quartz 5W- 30 which was the Mazda 5W-30 mineral oil recommended by Mazda.

You pays your money you takes your choice.

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Del.

[SprintRX8]

Rather than asking what’s the best Engine oil for a Rotary.

Wouldn’t it be easier to ask what Engine oil should I avoid like the plague in a Rotary.

[Hopkins]

I found it strange that the universal consensus is to go for oil that’s more viscous when hot, when the UK-spec twin oil coolers do such a good job at keeping the oil cool, it’s advisable to cover one up during the winter . I could be mistaken, but this makes me think that an RX-8 running 10w40 on an average UK day would be running less effectively in oil that’s too thick for the temperature.

My assumption was that, whatever the over-effectiveness of the UK oil cooling, the hot "30" viscosity was not high enough to prevent engine wear, so that is the part of the oil spec which should change. Since the oil is much more viscous when cool, I have wondered whether a 5w40 would be a good choice. That said, I used Essex Rotary's recommendation of Comma 15w40 mineral oil for most of the time I've had the car, but decided to switch to Miller's 10w40 semi-synthetic last year because I cannot resist fiddling with things. The Idemitsu is very appealing but seems way to expensive to change every 1.5-2k miles, as I do.

I also use 150-200ml of Idemitsu premix per tank, but I use 95ron petrol since I didn't notice anything except the cost when I played around with 97 or 99 back when I bought the car.

[R1DSO]

I found it strange that the universal consensus is to go for oil that’s more viscous when hot, when the UK-spec twin oil coolers do such a good job at keeping the oil cool, it’s advisable to cover one up during the winter . I could be mistaken, but this makes me think that an RX-8 running 10w40 on an average UK day would be running less effectively in oil that’s too thick for the temperature.

My assumption was that, whatever the over-effectiveness of the UK oil cooling, the hot "30" viscosity was not high enough to prevent engine wear, so that is the part of the oil spec which should change. Since the oil is much more viscous when cool, I have wondered whether a 5w40 would be a good choice. That said, I used Essex Rotary's recommendation of Comma 15w40 mineral oil for most of the time I've had the car, but decided to switch to Miller's 10w40 semi-synthetic last year because I cannot resist fiddling with things. The Idemitsu is very appealing but seems way to expensive to change every 1.5-2k miles, as I do.

I also use 150-200ml of Idemitsu premix per tank, but I use 95ron petrol since I didn't notice anything except the cost when I played around with 97 or 99 back when I bought the car.

Some more partially-informed thoughts from me on this, bearing in mind I’m not an engine builder and haven’t stripped rotaries to see the effect of different oil grade usage ...

5w30 has a viscosity of 5 when cold. A lower cold viscosity number is generally a good idea in cooler climates. If the oil is too viscous when the engine is cold, the oil will be too thick to get to where it needs to to lubricate everything. A lot of engine wear can happen in the first few mins of cold operation if the internals are not effectively lubed.

A viscosity of 30 is only reached when the oil is at operating temperature. However, if the engine is struggling to get the oil to operating temperature because the twin coolers are doing such a good job of keeping it cool in our mild climate, then the oil will have a thicker viscosity than 30 (or 40, or 50) and provide more resistance to the engine’s moving parts.

I’ve been topping you with Idemitsu, but have yet to swallow the cost of a full oil change at those prices. And, to your point, unless you have a SOHN adapter (not an option with an R3), regular changes with a cheaper OK oil are surely better than less infrequent changes with more expensive stuff.

I fill up with Shell 99 RON as I’ve read that pre-mixing reduces the RON slightly and V-Max also has cleaning additives in it. I haven’t tried 95 RON to measure the difference. I have found it’s noticeable on previous vehicles with finickity engines. For instance, I had a 660cc single cylinder motorbike that benefited from much smoother throttle response with higher octane fuel.

[ChrisHolmes]

From Mick Hurley of Hurley Engineering who have over 40 years of experience with Rotary Engines:

Mick revealed that by far the majority of engines and I did say majority not all that they have stripped the cause of failure has been the lack of correct maintenance the signs being severe blueing due to heat build up from lack of oil - the signs are immediately noticeable and as I said he does know what he is talking about, another failure contributor that they are now seeing more signs of is due to the failure of coil packs in the event of this happening if the condition is not immediately diagnosed increased fuel wash will soon cause a point of failure for the engine so make sure your engine is kept in tip top condition with respect to this.

One could argue that is either down to the recommended oil grade being used or the lack of frequent level checks.

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Del.

[ChrisHolmes]

Some people seem to believe that, because of the regular top-ups required, then changing the oil becomes less important, "as most of it has been replaced".
Many of these only top-up when the oil light comes on and then fill to the MAX (or beyond) "so that it lasts longer".

I could link to the FB posts that make this argument but I don't want to give them any credibility.

It just amazes me that there are so many RX-8 still running when these people are allowed to drive

[Delanor]

I have come across a couple of instances of RX-8 owners going to check the oil level to find no reading on the stick.

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Del.

[AndyBrad]

"5w30 has a viscosity of 5 when cold. A lower cold viscosity number is generally a good idea in cooler climates. If the oil is too viscous when the engine is cold, the oil will be too thick to get to where it needs to to lubricate everything. A lot of engine wear can happen in the first few mins of cold operation if the internals are not effectively lubed.

A viscosity of 30 is only reached when the oil is at operating temperature. However, if the engine is struggling to get the oil to operating temperature because the twin coolers are doing such a good job of keeping it cool in our mild climate, then the oil will have a thicker viscosity than 30 (or 40, or 50) and provide more resistance to the engine’s moving parts."

the viscosity of any oil when cold will be massively higher than when its warm. so you always want the thinnest oil when cold (if you like, lets say 0-5w) and the thickest when warm (lets say 40+) but the 2 are not comparable. ie a 5w40 should offer better protection than a 10w40.

[ChrisHolmes]

"5w30 has a viscosity of 5 when cold. A lower cold viscosity number is generally a good idea in cooler climates. If the oil is too viscous when the engine is cold, the oil will be too thick to get to where it needs to to lubricate everything. A lot of engine wear can happen in the first few mins of cold operation if the internals are not effectively lubed.

A viscosity of 30 is only reached when the oil is at operating temperature. However, if the engine is struggling to get the oil to operating temperature because the twin coolers are doing such a good job of keeping it cool in our mild climate, then the oil will have a thicker viscosity than 30 (or 40, or 50) and provide more resistance to the engine’s moving parts."

the viscosity of any oil when cold will be massively higher than when its warm. so you always want the thinnest oil when cold (if you like, lets say 0-5w) and the thickest when warm (lets say 40+) but the 2 are not comparable. ie a 5w40 should offer better protection than a 10w40.

You would not significantly have any more or any less lubrication difference between 5 & 10 when cold as both will produce sufficient pressure to create and maintain the hydrodynamic wave which the shaft journal effectively rides on in a plain bearing design. What you do have is less resistance to engine component movement, ie; the oil pump and bearings with the 5w grade which allows easier starting and reduces emissions as the engine is not working so hard once it fires up.

[PaulAV]

From Mick Hurley of Hurley Engineering who have over 40 years of experience with Rotary Engines:

Mick revealed that by far the majority of engines and I did say majority not all that they have stripped the cause of failure has been the lack of correct maintenance the signs being severe blueing due to heat build up from lack of oil - the signs are immediately noticeable and as I said he does know what he is talking about, another failure contributor that they are now seeing more signs of is due to the failure of coil packs in the event of this happening if the condition is not immediately diagnosed increased fuel wash will soon cause a point of failure for the engine so make sure your engine is kept in tip top condition with respect to this.

One could argue that is either down to the recommended oil grade being used or the lack of frequent level checks.

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Del.

Micks a decent guy, he rebuilt my engine, and is also a TV star (car sos rx7) when I originally went to his he was rebuilding a Citroën rotary engine.

[RenesisRaceBuggy]

An upgraded oil pressure regulator will only “allow” a higher maximum pressure at higher revs, at idle and low revs it has no effect at all and this is where the 5w30 oil pressure can be very low hence the preference of 10w40 oil. Edit by Chris.

Doesn't matter, all else being equal the flow is about the same (you'll get a tiny bit more with a the lighter oil, and possibly a lot more if it's cold and the bypasses are cracked open) .
The engine oil pressure you see on the gauge is just feed pressure, it's not the pressure in the bearings, that's generated by the bearing oil wedge itself, and is orders of magnitude higher than you see on the gauge.

[PaulAV]

An upgraded oil pressure regulator will only “allow” a higher maximum pressure at higher revs, at idle and low revs it has no effect at all and this is where the 5w30 oil pressure can be very low hence the preference of 10w40 oil. Edit by Chris.

Doesn't matter, all else being equal the flow is about the same (you'll get a tiny bit more with a the lighter oil, and possibly a lot more if it's cold and the bypasses are cracked open) .
The engine oil pressure you see on the gauge is just feed pressure, it's not the pressure in the bearings, that's generated by the bearing oil wedge itself, and is orders of magnitude higher than you see on the gauge.

It's not even feed pressure if your referring to the oem "guage" it's a low pressure light represented by a needle

[ChrisHolmes]

An upgraded oil pressure regulator will only “allow” a higher maximum pressure at higher revs, at idle and low revs it has no effect at all and this is where the 5w30 oil pressure can be very low hence the preference of 10w40 oil. Edit by Chris.

Doesn't matter, all else being equal the flow is about the same (you'll get a tiny bit more with a the lighter oil, and possibly a lot more if it's cold and the bypasses are cracked open) .
The engine oil pressure you see on the gauge is just feed pressure, it's not the pressure in the bearings, that's generated by the bearing oil wedge itself, and is orders of magnitude higher than you see on the gauge.

It's not even feed pressure if your referring to the oem "guage" it's a low pressure light represented by a needle

It’s almost a no pressure gauge as it’s either on, needle up the scale or off needle drops (and your heart) red light comes on and bank account hurts!

[RenesisRaceBuggy]

Haven't been able to trust manufacturer oil pressure or temperature gauges for a long time tbfh. The Germans started it, they wanted people to see how 'well built' their cars were so they locked pressure and temperature dials at the halfway point so they stayed steady, unless there was a serious/engine destroying condition in which case they'd flick to one end of the other.
The quest for 'quality' as the general public perceives it....