Best engine oil for the RX8 231?

[manuRx8]

I have started using 5W40 (Petronas Syntium 3000 AV). Cold it is fluid like a 5W30, while in hot it acts like a w40, reducing the shear effect in the gears of the stationary. I think that this covers the entire range.
It is true that the Petronas is a 100x100 synthetic oil, but since I have the Sohn installed, I have the oils separated and there is no problem with the waste.

[Daleevs]

Comma x flow is what i use i find it well thats what has been used by previous owner since rebuild

[Hopkins]

No-one else on 15W40? I'm using that as recommended by Essex Rotary (Comma X Flow, specifically).

[AndyBrad]

i would see an advantage to 5w40 but not 15w40.

i always thought comma was a really cheap oil?

[Hopkins]

It is certainly not expensive, but I think a lot of what you pay in oil for is longevity. When changing oil every 1500-2000 miles, this isn't such a problem. I stand to be corrected .

[RobinPZ72]

Hey each to there own!


5w is too thin thought we it doesn't let the oil system do it's job especially on start-up, but that's only what I think!

The best straight forward reading I've read is:

https://www.mazdatrix.com/faq/synthetc.htm


Robin

[bigpete8]

I thought a 5w XX gave more protection on a cold start than a 10w XX or 15w XX (the lower the first figure the more protection)

[RobinPZ72]

Yes in a piston!

I think with the thicker oils it might get up to working temp quicker due to it being slightly slower through the oil system.

[bigpete8]

Surely regardless of Rotary or Piston oil viscosity is the same thing.

From Mobil website https://www.mobil1.co.uk/learning-centr ... l-faq.aspx

"Most oils on the shelves today are "Multigrades", which simply means that the oil falls into two viscosity grades (i.e. 10W-40 etc). In a 10W-40 for example the 10W bit (W = winter) simply means that the oil must have a certain maximum viscosity or flow at low temperature. The lower the "W" number the better the oil's cold temperature performance.
The 40 in a 10W-40 simply means that the oil must fall within certain viscosity limits at 100°C. This is a fixed limit and all oils that end in 40 must achieve these limits. Once again the lower the number, the thinner the oil: for example, a 30 oil is thinner than a 40 oil at 100°C."

[AndyBrad]

sorry but i dont believe that to be correct.

a thinner oil will flow better when cold. It does not magically thicken up when its warm. Its still thin. we do not want the oil to be thick and flow less. we are looking at the second the key is turned.

The issue is the bearing tolerances are better suited to a thicker oil. the issue with the thinner oils is the protection on startup

[AndyBrad]

ah i was talking to the previous poster.

[RobinPZ72]

I totally agree the thinner oil flows quickly, but sometimes quickly isn't the answer! Anyhoo I'm sticking to what I think is ok and could be totally wrong, but hey these things happen!

Robin

[PeteH]

"Thinner is better"
"Thicker is better"
are not very logical arguments. "Correct is better" is the only valid argument. You have no way of knowing which is best, because none of us have done the work required to find out.

The optimum oil is that which provides sufficient viscosity to provide a hydro-dynamic cushion when at the dynamically varying pressures and temperatures seen within the bearing surfaces of the engine. The car manufacturers do lots of complicated calculations involving bearing clearances, bearing sliding speeds, bearing loads, local oil temperatures, local oil pressures, thermal expansion of all the parts involved, etc.. They choose their oils based on these calculations. There are many variables, and this is why every car has different oil requirements. We have no way of repeating these calculations, or critiquing them.

It is a truism that thinner viscosities flow more easily, and that reduces the parasitic losses of the oil pump, and therefore reduces fuel consumption (a tiny bit). For this reason car manufacturers will always try to push oil viscosity as low as they can get away with. The common belief in the rotary community is that Mazda pushed it too far with 5W-30, and that 10W-40 is a safer viscosity grade for the engine. This belief isn't based on firm, hard, scientific facts, but it makes sense, and no-one has shown any concrete problems with using 10W-40 except for an immeasurably small reduction in fuel consumption.

People can believe what they like, but none of us will ever have the evidence to be certain that our beliefs trump anyone else's beliefs.

[warpc0il]

My perspective here
viewtopic.php?f=6&t=78199&hilit=10w40

[bigpete8]

Agree that the current consensus is XXW 40 is considered better oil for RX8 than XXW 30.....
I do question is the Cold start performance of 10W XX vs 5W XX Mazda specified

Ideally I'd have thought a 0/5W 40 should be the best (only available as a synthetic i believe).

[PeteH]

Why?

[bigpete8]

Surely it would then be closer to the optimum viscosity/protection even at low temperatures.

The lower the "W" number the better the oil's cold temperature performance.

sae.jpg

[13Black]

Sozzz errbody I don't think lower/higher can ever be generalised as better/worse.

There is one ideal weight for a given situation, any deviation higher or lower from that becomes detrimental.</2p>

[PeteH]

0 is better than 10 isn't a logical argument. Nobody knows if 0W oil provides enough protection to a cold rex engine.

Your argument simply says that all car manufacturers should use 0W-60 oil because that's always got the "best" cold and hot performance. That's just not how oils work. Putting 0W oil in a rex could be far too thin for when the engine internals are cold, and the bearings are at their cold clearances. And it may not. Nobody knows.

[Dr. FrankenRex]

Saying that 0w60 oil is the best for EVERYTHING is like saying that ryvita is great for everyone. Every engine has different requirements, just like every body has different requirements. Some of these requirements we know, or can guesstimate, others we cannot.

As such any deviation, as said, from the manufacturer's recommendation is based on 'best guess' and experience from people who have rebuilt many many many RX8 engines over the years. Most engine builders in the UK recommend 10w40, which is why that's what goes in my RX8. Would I plop some 0w30 or 0w40 in it? No thanks.

That said, it doesn't mean that 0w-whatever isn't better than 10w40, just that it doesn't appear to be from the experience people smarter than I have.

[bigpete8]

Yet everyone seems happy to take it on faith that a 10w does less harm than the manufacturer specified 5w??
(I mentioned the 0 as it is possible but I use 5w30 semi)

i do suspect much like the 40 vs 30 argument it is very little difference in real terms if it is maintained well
(As a note I have read the comparisons to earlier rotary engines that use 10w40)

[RobinPZ72]

Yeah each to there own guys! If someone what's to put whatever w they want is upto them.

To get told I was the only one who used a 15w/40 was a bit weird though as some wankle restorers actually recommend it. One oil manufacturer for rotary engines makes there oil just that!

This must be the welcome beating for the knewbi!

Robin

[PeteH]

Yet everyone seems happy to take it on faith that a 10w does less harm than the manufacturer specified 5w??

Nope. I'm saying nobody knows. But we all have to put something in our engines....

Manufacturers will almost always specify 5W or 0W if the engine is used in very cold climates. I don't use my engine in arctic conditions, and I'm not bothered about fuel consumption. So I make my decision on whether to follow Mazda's recommendations based on my specific usage (note I'm not advocating anything, and I haven't said what I use).

[Eddie_r32]

I used 5w30 motul 300v, because RRP tell me to after they testee different oils and found that to be the "best". It's not cheap but it's a cool green colour

[Dr. FrankenRex]

Yeah each to there own guys! If someone what's to put whatever w they want is upto them.

To get told I was the only one who used a 15w/40 was a bit weird though as some wankle restorers actually recommend it. One oil manufacturer for rotary engines makes there oil just that!

This must be the welcome beating for the knewbi!

Robin

I dunno, the OP asked the question 'Best engine oil for the RX8 231?' - people have responded with what they use, and why and then the debate went on from there...

This isn't a 'beating for the newbie' or anything similar - if it was going to be that we'd all be recommending that the 'best' option for the OP is to use the below spec of oil:

S34RCH FNC5HUN