LarryL,
I did some research on your statement. Here is what I found.
A rotary engine is a modified four cycle engine that recommends the use of an API licensed motor oil for street applications. In a rotary engine, the oil lubricates the eccentric shaft bearings, thrust needle bearings and rotor bearings similar to a crank and rod bearing of a piston engine as well as being injected into the combustion chambers to lubricate the apex seals, corner seals, and side seals helping to create the sealing mechanism doing the equivalent job of the piston rings.
In the early development of synthetic oils decades ago, there were purportedly some seal compatibility issues. Today’s synthetic oils do not have the compatibility issues of the old oils.
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.
I heard that synthetic oil doesn't burn like mineral based oils and will coat the inside of the engine with deposits. If this was a problem with synthetic motor oils in general, then all internal combustion engines using a ‘synthetic’ would experience increased deposits on internal surfaces. The opposite is actually the norm. Conventional four cycle motor oils will typically leave deposits of carbon and ash when injected into the rotary apex seal, corner seal, and side seal areas.
The rotary engine metered oil pump should inject an ashless oil designed to burn in the combustion chamber and use a four cycle oil in the crankcase for the eccentric shaft, rotor bearings, and thrust bearings. For the street, Mazda simplified the OE system to use just one oil, that being a typical four cycle oil for both the e-shaft as well as the combustion chamber.
Hope this clarifies some things.
.
