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!!!
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