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.