It is a myth that good quality sythetics don't burn well or leave residue.

The 20W50 Amsoil has a flash point of 482F, and less ash (what is left after the oil is burned) than the famed Castrol GTX oils as well as any other 20W50 other than Redline. As a quick comparison, the ash left from Valvoline Turbo (conventional oil) is .99, while the ash left from Redline and Amsoil, both are less than .5 (with actually Redline showing less than .01 in many cases).

There are less additives and junk in the oil in most good quality synthetics than any conventional oil... which means fewer deposits.

That is why I recommend their use to people that have the extra money. Hands down, without any doubts, and proven over and over and over again, good quality synthetics burn cleaner, with less residue, and increased horsepower. The only thing that does not make them attractive is the cost.

I will say this again, for the people unable to search on this subject or too dumb to read the data sheets:

A good quality synthetic (like Amsoil, Redline, Neo, Royal Purple, Mobil1) is perfectly fine to use in a rotary. It is the poor quality ones (like Valvoine, Castrol Syntec, Havoilne, etc) should be avoided like the plauge as they do have the increased additives and left over ash that makes there use unsuited for a rotary or any high reving engines operation. Valvoline Synthetics and Castrol Syntec oils typically have a ash of 1.5...:o

And that hench is the problem... Mazda could not say, its okay to use Redline synthetic, but not Castrol syntec. Of course, Castrol GTX oils are probably the best conventional oils made, but Mazda couldn't say use one, but not the other... so they said don't use any.