This has got to be one of the most frequently asked questions ever!
Here is our answer:
The Rotary engine has an oil injection system that injects small amounts of oil into either the intake tract, carb, or rotor housing (depending on year/model). This is needed to lubricate the various internal seals and surfaces.
The injected oil MUST BURN, and must burn clean. The root answer to the question is that not ALL synthetic oils burn, and not ALL of them burn clean. (Idemitsu Rotary Engine Oil is the only known exception – see below)
The ones that do not burn accumulate until they foul the spark plugs.
The ones that do not burn clean can leave residues of various substances (like ash? plastic? non-organic sand?) that accumulate until the spark plugs foul, or a seal sticks — could be apex seal, side seal, corner seal, or oil control ring. The normal consequence of a stuck seal is an engine tear down.
In the many years we have been involved in rotary engines, we have NEVER had a problem with GOOD petroleum based oils. They work fine! They are less expensive than synthetics. (We use Castrol 20-50 GTX). They burn clean, etc. etc.
The problem with answering the original question is that it is NOT a simple yes or no. We DO simplify it to a “NO”, but that is because we do NOT know whether the specific brand of synthetic the customer has in mind will work. AND, if it does not work, how long will it be before the damage shows up, and how bad will the damage be? Maybe it will take 10,000 miles, maybe 50,000 miles?? Maybe the engine will fail due to something unrelated to the oil, and there won’t be enough left to determine that there was damage from the oil.
WE are not willing to take that gamble, are you ?
Then, take a minute to think of WHY you want to use a synthetic. If a rotary engine (properly maintained, oil changes at 3K intervals, etc.) can still be running fine at over 200,000 miles, the engine does not need any more cooling, the gas milage will not be any better, etc. etc. WHY do you want to spend more $$ and gamble on engine and/or spark plug damage?
We are not chemists, and we do not have the time, $$’s, nor inclination to do 100K mile tests of various synthetics in rotary engines.
The only exception to the above answer is with Idemitsu Synthetic Rotary Engine Oil. Idemitsu has been making synthetic oil specifically for the rotary engine for decades. They have only recently come to the US and we are happy to be able to sell it. We’ve known about it since the beginning, but until recently, it’s only been available in Japan. Click here to go to our oil page.
We DO use Neo synthetic in the transmissions and rear ends with great results.
Anyway – that is the MAZDATRIX version of the synthetic question.