Your friend is wrong hot or cold weather it does matter much for carb to head fitting
and i just checked the oil bottle its 20W-50 GTX
The old designed cd70e engine is more than happy with 20w-50 don't worry about oil
then i guess the problem is with the Carb.one question i have in mind if its a problem with piston or ring, does the bike goes to 100 110? runs for a month & half?