Well it's not been taken apart yet, so this is all a guess for now, but assuming it is a side seal failure I'm putting it down to because racecar.
It seems to have good compression so I don't think the lack of OMP caused any issue, and I'm guessing I don't have that much carbon build up. I'll report back once (or if) I get any more details after taking it apart.

Yeah, it’s blowing high pressure combustion gases into the sump. This is what blows oil up to the intake. Likely too hot of exhaust for too long leading to weakened side seals that can’t hold back the combustion pressure. It’s not just AFR, but EGT counts too. Especially long sessions repeatedly running WOT for long blasts like on a track.

Eric Meyer also theorized that one side seal failure mode is inadequate cooling of the interior of the rotors. The heat causes the springs to fail and no more sealing.

Based on who was building his engines I’d disagree.

Thanks for the details. after 150k km half of that is mostly going to racing, lapping ,timeattack and autocross races my S1 engine is starting to show low compression on one of the apex. I'm not premixing and still have the cat...

I was planning to put a S2 engine but just discovered the difference and that I would need to disconnect the OMP. I will try to get a remaf or new S1 engine to keep the reliability I'm used to. All the people that I know that are racing in my area are blowing their premixed engine after a few years.
I'm maybe the exception or the only one that still using the OMP and cat.

there were plenty of premix Renesis engines running multiple years in the Mazda Pro Formula race series that weren’t blowing up, try again …
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