This drove me nuts forever! I have a 98 YZ-125 - sometimes I would go to ride and 1/3 of the oil was missing. Sometimes none was gone. I checked - no leaks anywhere. I swore the right seal was leaking, but there was no other sign of this. I literally pulled my hair out over this and was ready to tear my cases apart when... a wise mechanic showed me the light! First, YZ 125's do use some oil - get used to it. The seal on the right side of the case is a sliding type and does not seal perfectly (all dirt bikes are like this). Second - you would probably notice less of a loss using non-synthetic oil over semi or full synthetic. This is a tranny - the synth isn't getting you much here - save it for the engine. Also, make sure to check your engine when it is cold. Don't top it off cold and then check it for loss right after a ride when it is hot (or vice versa) - oil will expand and contract based on temperature. If it is losing 30-50cc a ride - don't worry about it. Just stay on top of the oil and have fun! If you lose more and start seeing blue smoke and your jetting seems way rich / you oil-foul plugs like crazy.... then you can suspect the right seal.