RE: 90SP Muffler failure on run #4.
Mandatory castor is correct as per the instructions, but as a RCV rep now states on this forum Coolpower(synthetic only) is fine for the SP series.
Also, Bill at BJ's(RCV's authorized service ctr) prefers the Coolpower 15% and recommends it for the SP series. He indicated the 15% nitro improves the idle, and synthetic is much cleaner - it won't leave the castor stains all over(as mine did on the muffler), nor the carbon buildup. I burnt about a half-gal of Omega 10% thru my 60SP, and it appears to have loads of "protective" castor varnish now. So my plan is to switch over, since they now say synthetic is ok.
About a mth ago, my 60SP had to be sent in to BJ's for service after problems during run-in. He did several "revisions" apparently RCV still hasn't addressed. Although after several hrs run-in my muffler never came loose, that is one of them he did without my asking. I believe he brazed it in, or something to that effect. They also machined smooth the carb flange, and replaced a fiber washer w/metal lock washer on the carb i'm unable to see. I haven't yet tested it, but they state it now runs like a top(16 x 12 @ 5700rpm) without the stalling or rough-running issues i had. We shall see - hopefully within a couple wks when my plane is done. I'm not sure about revisions to the 90 or 120, but imagine they are similar? Other than my motor coming back with a slightly boogered outer prop lock nut and missing the crankcase vent cover(resulting in some oil coming out in the bag), i would not hesitate to contact him about getting any needed revisions done to them under warranty. Bill is very knowledgeable, helpful, and accessible. All it will cost is the shipping(mine was ~$10).
Hope this helps...
Mark