Do you really need to change the oil?, I'm just playing Devil's advocate here. Options

[VavAlephVav]

Really, aside from the cleanliness and longevity of the motor. How much of a difference does it make on the performance or power output of the motor if you have old burnt up oil that is broken down still in the motor?
Percentage wise how much power do you think you loose from having worn out motor oil that needs to be changed?

10%? 20%? more?

[Bitter]

If the oil is that bad then it could be contributing to the cylinders (well single here) washing down and not holding compression. People forget that oil coating the cylinder wall lubes the rings AND helps them to seal. A cylinder washed down with gasoline will have a substantially lower compression reading than one lubed with oil. Heck, look at wet vs dry compression test scores! Not only that but if that engine has a crank case breather that vents to the intake than the volatiles in the oil will gas out as the engine heats up and be creating a much richer mixture. I assume it's one of the new fangled EPA/CARB cert engines with a non adjustable carb? Those can be real touchy about old gasoline and dirty jets. No needles to adjust to get the best power which sucks, they always smell a little lean to me but that wouldn't jive with NOx targets, more likely they only meet emissions when they're new and clean, little dirt and they go lean. Run some gas with Seafoam (strong mixture) through it (maybe 1/4 tank gas and make it a 8:1 ratio gas-Seafoam) and see if she perks up any. Of course change the oil too and check compression, the cylinder could be damaged by poor lubrication and glazed up now. Wet and dry compression should be fairly close, within 20psi I think. It's been a while since I did any real work on lil engines. I shaved a head and cut some valve reliefs in a mower engine once, that was fun. Needed 89 octane or better not not diesel after that, but boy did it make easy work of thick wet grass! Little 2 strokes are much more fun to me, the smell great when they're tuned just right. Had a friend with a moped with a Minarelli engine on it, swapped it's cylinder from 50cc to 75cc and bumped compression from 8:1 to 14:1 with a Bosch Supercoil to jump a .045 gap instead of the stock .025 gap. Just the bigger spark made as much difference as the bump in CC and CR did! But that's cause the oil in 2 stroke mix is a real damper on combustion, but the extra compression really helps. 2 strokes are funny though, the more oil you add the leaner it runs so you can tune it and then tune it again!