I have been using the Mobil One synthetic that I get from Wal Mart. The 15/50 weight works very well in my YZ 250. Shifting is much smoother than anything else I have used. I dont think we are supposed to use the other weights of the mobil one. The 15/50 weight, with a red cap, is ok for our clutches. The others may make them slip.
About the bike, what is the worst thing that could happen? Sure, it could blow up, but it is not as if you have been using the piston for 14 years or anything.....uh, well,
Seriously, lots of times the bikes do give warning signs. Most of the time a person can hear odd noises or feel a power loss. It is not like a woman where one minute things are wonderful, then BAM!!!!!
I would rebuild it as soon as you can.