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Oil 101 for newbies.

Posted: Wed Feb 06, 2013 12:04 am
by Phil Bate
Oil 101 for Newbies

This guide is a mix of facts and experience collated from the forum, designed to help you decide which engine, gearbox and diff oils to buy for your RX-8 without trawling the many oil threads or starting a new one. Whilst it does not link to any particular supplier, it contains both the Mazda recommended oils/fluids, and some of the most common alternatives used by members of the club. Please note this does not constitute an endorsement or recommendation by the club itself.

Engine oil

Your engine continuously injects and burns small amounts of oil from the sump, by means of the OMP or Oil Metering Pump and a set of injector nozzles in the combustion chambers. For this reason you must not use (real) fully synthetic engine oils as they may not burn properly. Some oils however are marked as synthetic but are not really fully synthetic.

Mazda spec:
5w-30 oil meeting ACEA A1, A3 or A5. (10w-30 also acceptable in UK)

Mazda supplies Dexelia Ultra 5w-30, also available as Total Quartz 9000 Future 5W-30.

Other choices commonly used by members:
Fuchs Titan XTR 5w-30 (marked as synthetic but ok)
Castrol Magnatec 5w-30 - A1 Ford spec, not Vauxhall (marked as synthetic but ok)
Synionic 10w-30 Rotary engine formula (special mineral oil for the rotary)
Castrol Magnetec 5w-30
Havoline Energy 5w-30

If you wish to continue using the type of oil Mazda recommends for the RX-8 then any of the above examples will be fine and have been used extensively by other members.

10w-40 - alternative advice from rebuilders/specialists:

Some owners and specialists are of the opinion that Mazda specified a 5w -30 oil to improve engine economy at the expense of bearing longevity. Other Mazda rotary powerplants, some sharing the same bearings as in your RX-8, were spec'd with 10w-40 and have not suffered the early bearing failure we see all too often with the S1 RX-8 (R3 owners see the note below). In fact, rotary specialists recommend using 10w-40 and even stipulate it as a condition of engine warranty after a rebuild. You can read up about the opinions on this here, and about stationary gear bearing failure here.

If you decide you want to use a more viscous grade, some popular choices among members are:

Fuchs Titan Syn MC 10w-40
Castrol Magnatec 10w-40 (no ford/vauxhall differences in this grade)
Millers XSS/Trident Semi Synthetic 10w40
Shell Helix HX7 10w-40
Triple QX 10w-40
Tesco 10w-40 (Tetrosyl oil also marketed as Carlube)
Total Quartz 10w-40 (also available as 5w-40)
Mazda Dexelia 10w-40

R3 owners - If you have an R3 model then you have an upgraded oil system that Mazda fitted possibly to alleviate this problem. There are no instances of bearing failure on an R3 that we know of yet, but this could well be down to it being a younger model and having done less miles on average. Some specialists still advise using 10w-40 in the R3, and none have pointed out any problems with doing so.

Capacity / how much should I buy?

Owners hand books for the series 1 and the R3 quote oil capacity with filter change as 3.5 litres and 4.4 litres (approx`)

For each drain and refill you need 3.5 - 4.5 litres. It might make sense to buy 5 litre tubs as it will end up cheaper, and you will have some left over for topping up.

Link to guide for changing your engine oil here

What is a Sohn adaptor and why does it mean you can use proper synthetics?

A Sohn adaptor is a device invented by Richard Sohn that is fitted to prevent the OMP injecting engine oil into the combustion chambers, and replace it with a 2T (2 stroke) oil feed from a separate tank added under the bonnet. The device was originally intended for rotary engines fitted to light aircraft, but is gaining popularity with RX-8 owners.

You can only fit a Sohn adaptor to an S1 and not the R3, due to a difference in design with the OMP arrangement.

Once fitted, as your car is no longer burning engine oil from the sump you are free to use real fully synthetic oils to get the best protection for your engine (real fully synthetic oils can have higher shear resistance, better high temperature tolerance and can stay 'in grade' for longer). In addition, as 2T oil is designed to burn whereas 4T engine oil is not, there should theoretically be a reduction in deposits left in combustion areas. You will also no longer have to check the dipstick after every other fuel fill. Instead, you keep the 2T tank topped up or rely on the oil warning lamp if you fit a level sensor.

The devices are available here, and there is some discussion about them on this forum here, though more detailed discussion is available in the member's area.

Common 2T oil choices:
With a Sohn adaptor you use the same oil in the 2T tank that you would for premixing - see the Premix section below

Common fully synthetic engine oil choices when running a Sohn adaptor:
Shell Helix Ultra 5w-40
Millers CFS 5w40 / 10w-40
Millers Nano drive CFS 5w-40


Premixing is the practice of adding 2T(2 stroke) oil to the petrol tank before you fill up, to add extra lubrication for your apex and side seals (in addition to the oil injected directly by the OMP). Using a good quality 2T oil marked as safe for premixing with petrol (not just direct injection 2T oil) can help reduce seal wear and marginally increase compression, and there is some evidence that engines run with premix are cleaner internally when stripped down. Note that fully/ester synthetic 2 stroke oil IS safe to use, as it has been designed to burn properly.

2T oil specification:
When using 2T oil, look out for a JASO rating of FC (good) or FD (best)

Common choices:
Castrol Power 1 Racing 2t
Motul 710
Protek R


If you wish to get into premixing, start with 125ml per full tank of petrol. People will often increase this up to 300ml per tank for hard driving or track days.

Petrol choice when premixing:

Premixing can marginally lower the octane rating of the petrol in your tank. If you choose to premix you should seriously consider using a premium unleaded if you do not already - this will ensure your mix is as knock-resistant as possible, and the PCM can theoretically advance the ignition timing further for a better burn.

Gearbox oil

If you find it hard to select gears when the car is cold, or are getting any crunching then it's time to consider changing your gearbox oil. Changing to a good quality synthetic product can make shifting easier in general and help to remove any crunchy feel.

Mazda spec:
S1 5 speed gearbox: GL-4 OR GL-5 75W-90 (capacity 2.5 litre)
S1 6 speed gearbox: GL-4 75W-90 (capacity 1.75 litre)
R3 6 speed gearbox: GL-4 75W-90 (capacity 1.95 litre)

Common choices:
Castrol Syntrans Multi Vehicle 75W-90 Fully synthetic GL4
Redline Synthetic MT90 75W-90 GL4
Racing Syncro 70W-80 GL4 Synthethic Racing Gear Oil

Do not use Silkolene SILKTRAN SYN 5 75W-90 or Fuchs Titan Race Syn 5 75w-90

Guide/discussion about changing gearbox oil here.

Differential oil

NB Mazda and the oil companies do not specify a LSD oil for the Differential as the Diff` is a Torsen "type" and as such the gears will function perfectly happily on the specified "normal gear oil" a multigrade oil to GL5 spec` is acceptable to use but a specific SAE90 EP oil can also be used but it is down to the individuals choice.

Mazda spec:
GL-5 SAE 90 (capacity 1.3 litre)

Common choices:
Motul 90 PA Lubricant limited slip differential (LSD) Extreme pressure
Joe Gibbs "NASCAR" Differential Oil
75W/90 GL5 oil
90EP Hypoid gear oil.

Guide/discussion about changing differential oil here.


This guide is to assist owners/potential owners of the RX-8. Any inaccuracies or misrepresentations are unintentional. The author or the RX8 owners club cannot be held liable or responsible for any actions or consequences that may result from reading this guide whether consequence is resultant directly or indirectly. In any doubt always consider the services of a reputable vehicle inspector, UK Mazda Dealer or reputable Rotary & RX-8 expert notwithstanding any further liability shall rest with 3rd parties and not the aforementioned disclaimed parties.

Re: Oil 101 for newbies - work in progress

Posted: Wed Feb 06, 2013 12:10 am
by Phil Bate
Further reading

Here is a collection of threads (thanks to seanp for collating) - basically a trove of useful information expanding on oil theory and answering many possilble further questions you may have:

16 important OIL questions and answers

Oil Questions and Recommendations thread

oilman wrote:...Stick to Quality 5w-30.

Semi-synthetic, Part-synthetic or synthetic blend are the same things, don't use Fully Synthetic though.

You need to look for the oil that has the highest VI index, this is an indication of how resistant the oil is to thinning down when the engine is being run hard...

Post by AW8 - Response from Halfrauds

Post by moat - letter from Mazda regarding recommendation of Mineral oil usage in Rotary engines, including "early rotary" and RENESIS, side-seal materials, and reasoning behind 5W30 recommendation.

Post by Delanor - response from Mazda UK on use of Fully Synthetic.
Delanor wrote:...Here is the official line that Mazda are taking at this moment in time:

"Full synthetic oil has not been tested by the research and development team at Mazda Motor Corporation in Japan and for this reason, we are unable to recommend this oil"...
Post by oilman describing (and making further recommendations):
oilman wrote:Dexelia is Total Quartz 9000 take it from me, I have it on my shelves, just not got round to offering it via the Club yet.

Its an MC (Molecularly Converted) mineral oil, labelled as a synthetic blend (legally) so not a synthetic in the true sense of the word i.e. it's petroleum based not built by chemists in a laboratory from poly alpha olefins or esters. Having said that there are only a handfull of companies making "true" synthetics these days as the labelling means more profit on the petroleum based oils!

To be frank it would be more honest to label it a semi or part-synthetic but there you go, the marketing men have been meddling (see Mobil vs Castrol)....
Post from oilman - spoken to Silkolene (Fuchs') cheif R&D Chemist with feedback, mostly about seals and reasons for Mazda recommendations!

Post by oilman: Clarification of ACEA Specifications

Post by oilman: Information from Fuchs technical department on RX-8 oils (Recommending Fuchs Titan XTR 5W-30, of course ;-) ).

Immediate following post by oilman explaining Base-stock "Groups", defining "Mineral basestock" - "Hydrocracked" and "Molecularly Modified" etc.

PDF from Opie oils explaining [urlw= ... rovers.pdf]Viscosity Improving additives[/urlw], and how they can be a bad thing.

Re: Oil 101 for newbies.

Posted: Tue Jul 29, 2014 1:03 am
by warpc0il
Gumball89 wrote:Hi has anyone used mtf94 in ther 6 speed. It's
The 6-speed gearbox is very fussy regarding oil.

It doesn't like the cheap stuff that most (Mazda) dealers have on tap; these tend to make it noisy with a stiff change action.

It also doesn't like some oils "that exceed the requirements of GL-4"; these are too slippery and can prevent the synchromesh from working correctly, as it requires a controlled amount of friction to operate. In this case, if you "beat the synchromesh" then the gear teeth can get permanently damaged, making it increasingly difficult to select that gear.

I've been involved with Fuch Oil on detailed analysis of 'boxes that have been damaged in this way.

The only oil I'd currently recommend for the 'box is Redline MT90, based on my own experience and feedback from other members.

Mod bit: Added the quote from the original thread, to keep context about the inquiry.

Re: Oil 101 for newbies.

Posted: Mon Feb 20, 2017 1:47 pm
by warpc0il
While we're talking about oil, managing the oil level is critical.

Too low and you risk engine damage. :shock:
Too high and you'll get oil into the intake, contaminate the MAF and risk engine damage. #-o

One thing that owners often miss is the instruction in the manual :study:

The dipstick is designed to allow for checking oil level during a mid-journey fuel stop.
Owner Manual wrote:Inspecting Engine Oil Level
1. Be sure the vehicle is on a level surface.
2. Warm up the engine to normal operating temperature.
3. Turn it off and wait 5 minutes for the oil to return to the oil pan.
4. Pull out the dipstick, wipe it clean, and reinsert it fully.
5. Pull it out again and examine the level.
It’s OK between L and F.
But if it’s near or below L, add enough oil to bring the level to F.
If you check when the engine is cold, more of the oil will have drained down and you'll get an incorrect reading (too high).

For those with the later dipstick, this is marked to show how much needs adding.

This picture is from the US notice to dealers and is annotated in US Quarts
new dipstick.jpg
new dipstick.jpg (9.74 KiB) Viewed 11316 times
0.53 Quarts(US) = 0.5Litres

Given the potential MAF issue, I recommend aiming to keep the oil 0.25-0.5L short of the Full mark.

Re: Oil 101 for newbies.

Posted: Tue Feb 12, 2019 1:43 pm
by warpc0il
I'm not sold on the benefits of "long lifetime" oils, that are designed for extended mileages between changes, as I think it's better to change the oil more frequently rather than less.

However, we know that an engine oil change, with our twin coolers, only actually replaces ~60% of the total volume each time.

This means that, after the first change there's still 40% of the old stuff in circulation.
After the second change there's still 24% of the oldest stuff, plus 16% of the less old.

If you do a "double change" then you've got 76% fresh oil and 24% old oil.

Each subsequent change eventually removes more and more of the original oil, but the maths get too complicated for me to do in my head.

This also doesn't take into effect the oil that is consumed between changes and replaced with fresh - assuming you're not running a Sohn/COFS.

It's all a bit like finding Neanderthal genes in current DNA tests - there's always a bit left over.

The percentage per change can be improved if you do the change with a really hot engine and put a vacuum pump down the dipstick tube. This improves the numbers but doesn't change the fundamental process.

Some people have tried draining the oil coolers and this makes a more significant difference to the numbers but also introduces the risks of leaks or sudden failures, as hose joints are disturbed.

Don't ever be tempted to use "flushing oil" in a rotary. Many flushing oils are very poor at providing lubrication protection and are like giving your engine an enema. This might not be an issue in itself, unless they contain solvents that can attack the side seals or OMP injectors, but the calculations above show that you will never get all the flushing oil out and it will always degrade the performance of the oil in circulation.

Re: Oil 101 for newbies.

Posted: Mon Jun 10, 2019 10:17 am
by casey
We've had a few recent reports of 6 speed gearbox issues which may be due to the wrong specification oil being used. Several people (including myself, a few years ago) suffered from synchro problems when changing up from 3rd - 4th gear (so mainly seen on track). My gearbox was written off on the very first trackday after changing to a GL-4 and GL-5 spec oil :(

See this topic for the gory details of 3rd - 4th synchro failure:


I re-iterate Mazda specify using a gearbox oil compliant with a GL-4 spec ONLY, NOT a GL-4 and GL-5 compliant oil, the latter containing a lot more EP additive. See the first post in this topic for advice on tried and tested oils. Using an oil with GL-5 in the spec is risking synchro failure.

PLEASE NOTE that a lot of websites that recommend gearbox oils by entering your reg or car make/model will specify an oil that is both GL-4 and GL-5 compliant, which is NOT recommended by Mazda. Please check carefully before deciding to purchase.

It's also worth noting that using a GL-4 oil is the official advice from Mazda in the 2005 version of the Owners Handbook, which (for reasons unknown) showed a change to the spec from the earlier 2003-4 Handbook. One might speculate the change was due to real world experience on the the early models using a GL-5 oil?? But, that is mere speculation on my part, you can make your own judgement.

October 2005 Handbook (exactly the same in the 2006, 2007 and 2009 handbooks):
Oil page from manual.jpg
Oil page from manual.jpg (187.86 KiB) Viewed 5108 times

April 2004 Handbook:
Gearbox oil spec 2004.jpg