Would 85w_140 gear lube be ok running in the transmission? It says it's for off high way vehicles and manual transmissions.I can get a 2 gallon jug for free from work still sealed but not sure if it's too thick for my 500 or 250.

Ok bc I've been running 80_90 in the 250 bc I had a blaster and my kdx the clutches were slipping with the lighter oil which was 10w30 and switched to the heavier and helped so I just ran it since but I didn't know it was bad for the clutches but it could be that it helped with those bc they didn't really put out much power anyways.I guess I won't take the jug.Can you use just regular motor oil or does that cause clutches to slip bc of the detergents in it? That's what I've heard which is part of the reason I switched to heavy oil.

I am currently trying out some motor oil that I purchased from Wal-mart.  It is brand name Accel and it is for pre 1988 vehicles only and has a 10w-40 SE label.  Also says something about high zinc content.  It is super cheap so you can change it frequently.  Don't have enough hours on it to know how good it is but the clutch is not slipping

I've got to switch to something thicker than type f ATF fluid because it's seeping out of my seals. Until I can replace the seals what's the best gear oil to run in your opinion Motorgeek? Price don't matter and neither does change frequency I've been changing my gear box fluid after every two rides.

Just change the seals or replace the surfaces the seals make contact with.  You are wasting your time trying to stop the leak by using thicker oil

Overfilling by about 400ml each time I changed my fluid, do you think that caused my seals to fail?

Not if the transmission vent is not plugged.

The only thing that causes a seal to leak is a bad seal, damage to the shaft that the seal is sealing against or the shaft is so loose in the hole in the case that the shaft can move to the point that the seal is no longer touching the shaft around it's full circumference.