JJ-
In my case, it was the first race of the 2005 season at GJMS. The other person who described a similar experience was the following race at Bandi. I’ve not heard of problems since then on the Rotax boards, but that’s probably because Rotax changed oils.
Last season we ran Motul for Rotax Max Challenge races (as spec’d by the RMC) and VP RS-7 for CSC races. (So aside from protection issues, it’s also a nuisance to have to have two different fuel mixes from weekend to weekend.) However, we generally ran more oil than Rotax calls for as extra insurance. That is, while Rotax officially calls for 50:1, we’d generally run more like 44:1, and even 35:1 for really hot days. I don’t know if that really helped, but it seemed like prudent insurance.
As for warranty coverage, Doug or one of the other Rotax service centers can give the authoritative answer on whether Motul is required in order for the warranty to be effective.