Q:  Can I use synthetic oils in my engine?

The use of synthetic motor oil inside a rotary engine is one of the most debated topics since the birth of the rotary engine itself.  Mazda's version of the rotary engine requires internal lubrication; Mazda engineers designed an oil injection system (and pump) that injects engine oil into the combustion chambers to fullfill this requirement for its need for internal lubrication.  It is this fact that triggers the debate on synthetic oils.

With this stock oil injection system intact, oil from the engine is used to lubricate the internals.  Synthetic oils have a higher "flash point" than mineral-based oils.  This means that is takes higher heat temperatures to burn the synthetic oil.  With the higher flash point temps, there is a chance of more by-products of the (synthetic) oil staying behind in the engine after combustion.  While core combustion temperatures can reach over 1,000°F, internal combustion (metal) surface temperatures are not.  Is it this fact that synthetic oil by-products can adhere to internal engine parts that can affect performance.  Check out Ed Hackett's Oil FAQ on various oils, flash point, etc...
Click here for the Oil FAQ page!

We've seen a 13BT out of a 1988 FC turbo produce significant deposits adjascent to the spark plug holes on the rotor housings.  The owner was using Amsoil 20W50 exclusively with very low original miles (about 30,000 miles).  This minimizes the possibility that the deposits were made prior to the Amsoil 20W50 was being exclusively used.  These deposits causes the apex seals to skip when then run over the affected area.  Skipping apex seals will cause decreased engine performance by degrading sealing between the combustion chambers or rotor faces and the rotor housings.  We never even bothered to look for deposits within the grooves of the rotors, but I think it is safe to assume that they are there.

We have never seen nor heard of oil lubrication problems due to (mineral) engine oil quality.  If you stick with a quality, name-brand mineral engine oil, there's no reason to switch to synthetic engine oils.  We prefer Castrol GTX 20W50 mineral-based engine oil for those still using the stock oil injection system.  Mazda's official stance on not being able to recommend synthetics in their rotary engines is that they were not able to test all synthetics oils available to pass their approval.

If you do insist on running synthetic engine oil, please inquire with the oil manufacturers for recommendations in a rotary engine.  We only know of Royal Purple that okays it's synthetic engine oils to be used in rotary engines that use the stock oil injection system.

For those owners who have eliminated the stock oil injection (and using pre-mix), the use of synthetic oils in the engine is fine.  There should be no injection of the synthetic oil into the combustion, and no chance of the abovementioned problems associated with the stock oil injection system.  An unofficial comment from one of the Mazda engineers working on the rotary engine projects was that pre-mix is a superior method of internal lubrication in the rotary engine, but it would've been too much hassle for most typical consumers to worry about refilling an additional fluid reservior; simplicity overruled superior performance in this case.
Click here for out FAQ Pre-Mix page!

Some cavaets on using synthetic engine oil in your vehicle...  The typical synthetic engine oil molecule is smaller than it's mineral-based engine oil counterpart.  With the smaller molecule, synthetic oils can cause more oil leaks from the engine.  Also, when breaking in a fresh engine, it is recommended to initially use mineral-based engine oils.  Synthetic engine oils can be too slippery to cause proper break-in, and the use of mineral-based engine oil during break-in ensures quicker mating of the seal contact surfaces.

Synthetic engine oils are a valid argument here due to the use of turbos.  It has always been a recommendation to always use synthetic oils with turbos to help with turbo lubrication and high heat capability associated with turbos.  With the stock turbos, the use of a water/coolant circulation through the center section helps reduce oil problems associated with turbos; the water section helps prevent oil "coking" from the high heat.  Turbo timers will also help with cool-down, if you're paranoid about oil abuse.  Common sense is probably the best safety factor to prevent any oil problems from the turbo - don't turn the engine off after max boosting the past several minutes!!!

Questions?  Comments?  Send email to:  reted@fc3spro.com