i think he meant 15% nitro. but still it would have plenty of oil. i'm assuming that is a 2 stroke. never looked inside a 4 stroke rc engine to know what they look like. i dont know the differnce b/w 2 and 4 stroke fuel. if i had to guess the 4 stroke would have a little more oil because of more moving parts but i could be wrong. i'm sure someone will chime in that knows more about this than me. but i just always stick to 2 stroke fuel with 2 stroke engines and when i get a 4 stroke i will just stick to 4 stroke fuel with 4 stroke engines. but moisture could have gotten into your bearings and rusted and when they got broke loose from the starter and ran a little while all that rust ran through your engine. that greyish color you saw was more than likely metal shavings. which was probably from bearings and other stuff grinding down. i've had several bushing engines lock up on me and i had a silverish looking oil instead of dark brown/black oil on my planes. i'm sure there wouldn't be that much differnce. metal shavings are metal shavings.