I was reading the current issue of MazdaSport and they had a nice article on Synthetics and the Rotary.
They say that the reason why mazda recommended mineral oil was because of the whole spectrum of different synthetics each with their own qualities and that the rotary required an oil that is "totally emulsifiable in gasoline and that it burns 100% clean". So as far as mazda themselves, they haven't tested any brands of synthetics, I'm assuming that's why they don't want to give a general "thumbs up" to ALL synthetics all with their own additives and processing.
However, synthetics (if you choose the right one!) have quite a few interesting benefits.
Rotarys are susceptable to cold start oil starvation, synthetics do a better job clinging to engine parts after shutoff to lubricate for the next cold start than mineral oil.
More importantly, synthetics have a superior thermal stability. This is extremely important for Rotarys because they "experience spots of considerably higher temp. than piston engines. In piston engines, exhaust temp. run around 1250 F, where as in the rotary, it's closer to 1900 F. Also where the reciprocating parts of a four stroke piston engine go through 3 unpowered (and therefore cooling) strokes for every powered one, a rotary only have 2 per cycle.
In general, synthetics tend to start suffering thermal break down between 600-700 degrees F. For natural (mineral) oils it's around 350-400 degrees F, which is cutting fairly close for an engine design (rotarys) that may see internal hot spots of 330 F in normal use (which is why rotarys must change their oil religiously if they are usuing mineral oils)! Synthetics makes even more sense once you talk about turbocharged applications where localised hot spots up to 450 F in normal operation and 650 F after shutdown (without turbo timers I guess)." (Qouted from Mazda Sport article May/June issue)
The final thing to take away from this article is that it says Mazda might not cover engine failures due to synthetic use (how they can prove that is another wonder), and owners might be stranded to deal with the oil companies (reputable ones these days will stand behind their product and cover repairs). Last thing is that the only synthetic oil the whole article claims to have tested extensively and found perfectly suited to rotarys is Red Line (though I imagine Royal Purple has also done their testing as well).

 

Nice post, but this has all been covered in the past

 

oh ok ops
wondered if anyone read the same article :p

 

I read it, but found it extremely annoying that after all that, the author didn't really recommend ANY suitable synthetic oil! Royal Purple was discussed, but he didn't summarize and say "THESE oils are OK".

FWIW, I use Castrol Syntec - according to Mobil (lawsuit a few years ago), Syntec isn't really synthetic because it doesn't use the same PAO synthetic base stock, but is made from triple-refined crude oil. In effect, it's just extremely good mineral oil with much of the same characteristics of synthetic. BUT since it's still mineral oil, it has to be safe for Renesis no matter who you believe about Renesis/synthetic use! :D

Regards,
Gordon

 

I just put mobil 1 in. (mainly due to availability) I plan on changing it & filter every 3000 miles. All my previous rotary's used Castrol GTX. No problems but they did get power loss as they got older.

Anyone know anything bad about mobil 1 when compared to other synthetics (not dino's). I hate to have to buy my oil off the internet or at a specialty shop. Anyway, has anyone who changes there oil regularaly had a bad experience with it in a post 1990 RX-7 (or RX-8).

 

well I've been running mobil1 for 6000kms now and no adverse effects noticed other than a superbly quiet idle (placebo effect?) but yea I plan to switch over to redline next oil change
LOL yea that article was kinda sitting on the fence and saying usuing synthetic COULD void warrenties on engine repairs etc BUT, synthetic is good for the engine in such and such way, so basically they said synthetics are good, but good luck if anything goes wrong!

 

Same as Gordon I use castol syntec. I did not realize that it was not completely synthetic, but now that you mention it I guess I am glad that I do use it.

 

I have used Mobil1 in all my rotaries since 1999.....never had a hint of the supposed problems with synthetics....