lean after rebuild

[twinrock101]

Hey guys been awhile..i just rebuilt my dt200 ..top end only, basically it had a last over size bore on it which was damaged do to pump issues, that was 10 months ago..and was riding the bike ever since..still had plenty of effortless power.. i had a fresh top end waiting 1mm os ..the other one was getting very noisy so i swapped it out..bike started second kick running soo smooth and quiet..took it for a run and noticed it just didnt have the same bottom and mid pull as before...and has a lean feel to it..not a racing revs air leak lean but as if leaned out the needle..the only other thing was i changed oils.. i went from a cheap outboard oil..to elf moto 2.. the obvious thought would be an air leak base or head gasket..but from my experience head gasket leak lack of compression, hard to start base gasket high racing at idle. carb or manifold was not removed as crank case inducted , no need to remove ... could it be oil..? Thicker oil would possibly lean it out.. the bike runs perfectly with no misses or issues butter smooth...any thoughts thanks