ok everyone thank you for all your help. you have given me great information. it wasn't going back to 20-1 that worry'd me it was what other oil to use at that ratio but now i can go and rebuild it and hope fully it will last for a few race seasons longer 
The stock jetting and timing on the '82 RM250 are very good but you may need to richen it with a highly oxygenated fuel or lean it out for a non-oxygenated race gas. As long as you use a thinner, injector compatible oil like Castrol Power 1 TTS, the 20:1 - 25:1 ratios will work perfectly. The bike likes leaded fuel but over 100 octane is a waste. 100LL avgas works great if you can get it.
