Can I use synthetic oil in my RX-8?

This topic is hotly debated. The RX-8 owner's manual says to use 5w20 oil that meets API and ILSAC GF-3 specs. It doesn't specify oil type.

The reason why people still think rotaries and synthetic oil don't mix, dates back to the 70's when synthetic oils were new on the market. Rotary engines have a set of oil control seals in the sides of the rotors which are designed to keep oil from the lubrication system out of the combustion chambers. Each seal consists of a metal scraper with a rubber o-ring within. The o-ring was not compatible with some synthetic oils and sometimes the oil would attack the o-ring causing them to break down.

The RENESIS engine in the RX-8 has completely new o-rings that have improved longevity and compatibility with synthetic oils. Mazda does not prohibit synthetics from being used in the RENESIS engine, but recommends standard mineral oil. This is not limited to the RX-8 however, Mazda recommends mineral oil across the board for all their cars.

Here is some food for thought:

The Mazda Factory racing departments recommend and use �synthetic� oils including the winning 1991 Leman�s 20-G 4 rotor Mazda 787B.

MazdaComp USA printed manual (now Mazdaspeed) recommends the use of synthetic oils for racing conditions.

What is synthetic oil?

Synthetic oil is made by synthesizing chemical compounds that were not originally present in crude oil. The new base oil is then combined with a series of additives. The main difference lies in the fact that synthetic motor oils are created utilizing a specially "synthesized" base oil where the size of the oil molecules are all of an ideal weight and of a consistent size. While a conventional motor oil is made up of different molecule sizes which are mixed together, along with various waxes and impurities, a fully synthetic oil is made to provide a much purer base oil, with less waxes and with a uniform ideal particle size to help increase the oils viscosity level. The additives which are used in the production of synthetic oils can help to create an extremely stable engine oil which will sustain the correct viscosity levels across a large range of temperatures.

So what does synthetic do for you?

1. Superior friction reducing between sliding surfaces.

2. Lowers engine temperatures. Oil accounts for 1/3rd of a rotary's cooling. You will typically see ~10 F lower temperatures with synthetic due to reduced friction between sliding surfaces.

3. Reduced foaming. Foaming is caused by air bubbles that become trapped in the oil. The bubbles prevent heat from escaping the oil, causing temperatures to rise.

4. Burns cleaner. There is reduced carbon deposits because of the ash less base. Synthetic oil also has less impurities and will burn cleaner than conventional oil. It's all but required for new diesel engines because conventional oil has much higher sulfur content.

5. Synthetic oil lasts longer then conventional oils in extreme heat conditions. This becomes especially important for FI cars.

A rotary engine can be damaged by heat very easily. Oil temps should never exceed 250 F.

What are the drawbacks to using synthetic oil?

1. Main drawback is cost. Synthetic oil can go for more than twice the cost of conventional oil.

2. Not suitable for breaking in your engine. Synthetic oil's lubrication and additives will inhibit the break in process. Your first few oil changes should be with standard mineral oil.

3. Oil leaks can occur when switching to a fully synthetic oil with an old engine. The introduction of a synthetic oil may cause the detergents in the synthetic oil to break down and clean-up waxes and sludge which was maintaining the seals in the engine. Decide if you want to use synthetic sooner rather than later.

Questions and answers

Q: If synthetic oil resists heat better than conventional, doesn't that mean it's not going to burn in the engine as it's injected?

A: You're not burning quarts of oil at a time. The MOP is injecting tiny amounts of oil to lubricate the seals. At that ratio of gas:oil, the oil mixes with the fuel and burns quite easily in the 1600+ F temperatures in your engine.

Q: Why does Mazda not recommend synthetic oil?

A: Synthetic oil performs better than non synthetic oil. This isn't up for debate, it's a fact. With that said, Mazda doesn't recommend it, and no one knows why.


Some theorize that:

1. Mazda has no way to test all synthetics to make sure they're okay for use, thus they take an all or nothing approach.

2. Another theory states that one brand of synthetic oil leaves more carbon after being burned than it should. This in turn causes carbon buildup in the engine. In order to avoid legal implications from naming a specific company not to buy from, Mazda simply states no synthetics. Since I'm not Mazda, I can mention to you that Mobile 1 is the synthetic oil in question. Please keep in mind this is an unsubstantiated rumor.

Q: Does "FULL SYNTHETIC" really mean the oil is synthetic?

A: No. Oils such as Castrol Syntec say "full synthetic" on the bottle, but are derived from crude oil. In 1997, Castrol changed the formula of its Syntec "full synthetic motor oil" and removed PAO from the base stock. It's now based off of hydrocracked mineral oil. This move saved Castrol's production costs by 50% while they continued to charge normal synthetic prices. Moral of the story: Make sure you do some research before buying. Castrol isn't the only one doing this.

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User Comments

incredibly informative
Does "synthetic base" mean its fully synthetic? I'v seen semi synthetic but i chose "synthetic based" thinking its more synthetic than semi? I am using Shell.
I tend to buy oil from my local Mazda dealer that specifically made for the RX8 - or so the bottom tells me. Is that stuff mineral ?
The current wisdom in Dublin (Republic of Ireland) favours mineral oil over both synthetic & semi-synthetic oils in your RX8. The seals are far less likely to be corroded by mineral oil with an obvious advantage to longer engine life. The reports of the sooted/carboned-up interiors of engines that have used mineral oil is not borne out in practice. The source for all this comes from an Irish Rotary engine specialist who has rebuilt dozens of engines (RX8, RX7 and Cosmo).
Crackjar synthetic base means that "50% or more" is synthetic and the rest could be additives or mineral oil. if you want a good full synthetic oil Pennzoil Platinum has a higher rating than Mobil one. but if you are looking for one of the best oils i would go with Pennzoil Ultra so far has the highest rating on the market
Please do not forget to mention that not all synthetic oils should be used. Only ethanol based oils can be used. This is the same reason why Mazda recommends mineral oil (Dino oil) since the oil needs to be burned during combustion due to spit back. The only oil ever created for the rotary engine was created by Idemitsu for the 787B and it was an ethanol based synthetic.
look, i have listened to that debate for years now!I have had 4 rx-7s and1rx-8 i have been runniing muiltiple weights of mobil1 for all of my cars with no problems!
Hi I am reading all kind of comment about what oil to used in the rx8 ,I am from south American Guyana and I have a rx8 04 .and all the rx8 here used 5w/30 shell and working well .
I just bought my rx8 with 70,000 miles on the clock, since I have absolutely no idea which oil any previous owner may have used I have plumped for Castrol Magnetec 5w30 synthetic. The main reason being that Mazda Dexelia 5w30 is quite dificult to get hold of. I have read much in the last week or so about Synthetic oil invalidating warranties ect. This does not affect me as I don't have a warranty, however, I have no desire to ruin the engine on my new pride and joy. I would appreciate any further comments or input on the subject. I shall monitor this page for reply's over the next couple of weeks.
Your information contradicts documented reasons I have read as to why Mazda does not recommend the use of synthetic oils in its rotary engines. Please comment on the following: Synthetic oil is engineered primarily to lubricate, not to combust; however, rotary engines are designed to inject oil directly into the combustion chamber, where the oil mixes with fuel vapor to lubricate all areas and components of the chamber in which the rotors revolve, especially the seals. Synthetic oils, when ignited in a rotary combustion chamber (but unlike when synthetics lubricate cams, pistons, valves, etc., in conventional engines), may leave post-ignition ash and other deposits that can harm a rotary and its seals. If only partially burned in rotary ignition events, conventional, non-synthetic oil may leave behind some residue, but not the kind of potentially harmful ash, etc., that synthetic oil would leave. Because you do not point out basic oil-usage distinctions between rotary and conventional reciprocating internal combustion engines, I tend to lose faith in the information you offer here. If the latest generation of synthetic oils indeed do not leave extraneous, harmful residues or compromise lubrication when used in rotary engines, the manufacturers themselves, not their PR surrogates online, should state so unequivocally in their product labeling, and they should provide Mazda's endorsements.
excellent information, and no significant or proven cases to show otherwise!
Roger, I wonder why you have had 5 rotary engines :P
Another contributor to carbon build up on these rotaries is driving them too gingerly. They are made to rev so rev it and enjoy it.
Well, I've been running 5/30 Dino and changing every 3000 miles. My engine just blew at 42000.
I have owned a Mazdaspeed Protege, Mazdaspeed 3, Mazdaspeed Miata, and now an RX 8 R3. In all but the R3 i uaed Mobile 1 full synthetic and was very pleased. I currently use regular Mobil 5W/20 in my R3. I would personally like to know if I can go back to Mobil 1. I have about 40,000 on my RX 8 with no issues. But would like it to run smoother above 8,000 rpm. And remember, the R 3 has a dry sump oil system with twin oil coolers up front. Very different than a regular RX 8.

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