This is one of the never ending debates. Well I guess I should first address the myth that synthetics don't burn as cleanly as dino oils, thus leaving behind harmful deposits. I believe this to be untrue. Both have similar ash contents, which are mostly the result of zinc that is added to prevent seizing. Please review the link: http://www.vtr.org/maintain/oil-overview.html Some people also argue that synthetic flash points are higher so they won't burn as completely but a flash point is the point at which the oil catches fire at a certain temp. In the engine there is fire and all oils will burn. There is a potential drawback though, which was the original reason Mazda said not to use synthetics. Have you ever heard that when synthetics are put in some cars they start leaking oil? Well, it's true. The reason is that regular dino oil contain impurities, which cause various seals to swell like the oil pan seal. In a rotary there are different seals in the engine that must swell to maintain component integrity. I believe today's synthetics are adequate and many people use them with no ill effects. To back up some of my claims I present this posting to the big list from David Lane:
After many years of discussion on this topic, I finally
ran across something from a manufacturer that addresses the history of the issue
directly.
My source was, believe it or not, a wall in the office at Sun Auto--a junk yard
specializing in 1st and 2nd gen RX-7s. It is in the form of a letter written on
Amsoil letterhead, and signed by someone in the
Technical Services Department. The letter was written to a private individual,
but it looks like a form letter, and I saw it in a public place, so there should
be no harm in paraphrasing it here. I am
working from a copy.
The letter says the reason Mazda did not recommend synthetic oils was due to
problems with the rotor seals (part numbers 0820-11-341 and 1202-11-343). The
problems were apparent with "at least one" synthetic oil.
Amsoil had been sponsoring RX-7 racing since the '70's, and thus had always been
concerned with this.
When Mazda came out with their caution in 1988, Amsoil tested for seal
compatibility.
The letter implies that the seals in question were designed to swell as a means
of replacing worn material and maintaining component integrity. The idea was to
control this swelling within an acceptable range, which Mazda specified as being
between ten and twenty percent.
Amsoil used Pennzoil 10-30 as a reference and measured an 18.5 percent swell
rate. Several Amsoil multi-weights were tested and found to range in "swell
rate" between 14.5 and 17.3 percent.
The following line is a direct quote:
"Since that time, Mazda has rescinded their recommendation not to use synthetic
oil in the Rotary engine."
End of paraphrase.
Now, this letter is dated September of 2000, and the notion that Mazda didn't
caution against using synthetics until 1988 was enough to send me scurrying to
the owner's manual for my 1985 GSL-SE. No mention of synthoil there--nor in the
shop manual. The only requirement is an SD, SE, or SF oil.
The last line--that Mazda had recinded their anti-synthoil stance--was also a
shocker. I would have assumed any information like that would have hit this
list like a clap of thunder. Of course, I am in no position to confirm the
assertion, or to argue with it.
Nothing above is going to change anyone's mind about the issue, but it does tend
to reinforce some of the points made in past discussions, and to put things in
historical perspective.
If anyone has a direct source to verify the notion that Mazda currently has no
prohibition against using synthetics, I would like to know about it. Last time
I saw a Mazda reman in a crate, I THOUGHT I saw a tag on it with a warning that
using synthoil would void the warranty. (Note: these engine seem to have about
a dozen tags tied on them when delivered. You'd think someone would save time
and energy by supplying a detailed instruction sheet instead.)
And, of course, there are no rotary cars currently in the United States with
original engines still under warranty, so the point is moot from that
perspective. We'll have to see what happens when the new rotary cars hit the
showrooms.
Best wishes,
David Lane
Now that I have established that synthetics aren't harmful in my opinion I
should now address if they offer any benefits. After all, they do cost more and
oil related failures in rotary engines are virtually unheard of. There are 4
main benefits to synthetics.
#1 Slicker and thus less friction
#2 Lower oil temps, or so I have heard
#3 Doesn't turn acidic over time
#4 Maintains viscosity better
#1 I have heard many people cite this as a main reason they use synthetics. I
always ask, "where is there less friction?" I mean there are far fewer moving
parts in a rotary engine so where does the benefit occur? I must therefore
dismiss this as a reason to use synthetics.
#2 Lower operating temps is a good thing for sure but I have tried synthetics
and have not reproduced this claim. I have heard it from some reputable sources
though. The cause is cited as either less heat causing friction (I doubt it) or
better heat transfer properties. I plan on getting an oil temp gauge soon which
should provide me with a clearer indication if this claim is true. I should note
that the claims have always been differences in water temp though.
#3 A great reason and very true but if you change your oil every 3K or less this
isn't much of a concern.
#4 Another good reason but without the other reasons holding true it doesn't
make much of a difference in a rotary.
So after all of this I guess I still haven't answered your question. Well
personally I think if you were going to use a synthetic I would go with either
Mobil1 or Royal Purple. These are the two that are recommended most often by
Rotary experts and for good reason. I have looked at the properties of these two
oils compared to others and they are the best. Mobil1 looks to offer the best
protection while Royal Purple is the only one that is proven to increase hp in a
rotary. (See Racing Beat's web site.) So personally I don't often use synthetics
and when I do it's only for when I want to test it or if I'm racing. Otherwise I
use dino oil.