using 2.6 ounces oil per gallon fuel mix = 49:1 ratio
using 4.0 ounces oil per gallon fuel mix = 32:1 ratio
I know you guys (Duck) mean well telling him that he is possibly running too lean and to add more oil to his fuel but more oil in the fuel will in actuality cause a more lean condition. I agree - it is important to have enough oil for lubrication - don't get me wrong. I typically run about 40:1.
It is common to "fine tune" jetting by changing fuel/oil ratios. Adding a little more oil to get a more lean condition. Lean refers to the Fuel to Air ratio. The more oil in the mixture the less fuel there is with the same volume of liquid.
Obviously you carbs jets & air screw have the greatest effect on this.
A higher octane could help you but it might only be masking your problem. As your engine heats up things expand - your compresion increases. A lower octane fuel will detonate prematurely if the compression gets too high (usually sounds like knocking). This is why you have to use higher octane in high compression racing engines. Most MX bikes with any engine work are running 118 octane race gas.
Have you checked out the wear on the rings/piston and the cylinder wall? Could have a slightly warped cylinder wall or piston. As it heats up there would be more friction.
If there are any air leaks - this too could cause you to be too lean. check for leaks around the carb coupling and by the reed plate. If there is a leak add some high temp silicone to seal (and not just on the outside - take the thing apart). One way to check for leaks is to let the bike sit and idle. while it is idling take a propane torch (off) and release the propane gas at different points around the engine (cylinder head, any seams & couplings) if the rpms increase as you do this and decrease when you move the propane away - then you probably have an air leak.
Hope some of this helps. I have a 103 SP too. Runs great.