I bought a used 2015 honda foreman fe1 about a year ago and rebuild the entire transmission, the atv was riding perfectly until I relized i had a oil leak and my dad poured 20w-50 in it just for a days ride, but then it started slipping 5th and kinda jerking and popping into 4th, it wont do it unless ur above like 30mph and i ordered a set of friction clutches for it. would anyone know what my issue might be? my entire transmission is new.

 

Your description is a bit vague, if the clutch slips you get some drive, if it pops out of gear you either get no drive or it jumps into the next gear, this indicates worn dogs and/or a bent selector fork. The water cooled engines are a lot less prone to gearbox problems than the old air cooled 500 Foreman, but those used on asphalt a lot, do seem to be prone to slipping out of 5th.

The wrong oil can cause clutch slip, I had big problems with a 400 King Quad after our oil supplier changed oil makes during the pandemic. The supposedly "identical" replacement oil made the clutch slip.