My girlfriend's son has a '87 Isuzu Trooper with manual transmission issues. He bought the truck a few months ago and today he says the trans starts making noises. Sure enough, bad noises whenever you're on the gas except in 4th gear. I figure syncros or bearings. I say lets check the oil level first. The usual fill plug on the side of the trannny. I take it out and I bet a gallon and a half come out before the oil level gets below the fill plug
How could this happen? Could a rear main seal leak on the engine fill the transmission up to the top?
Looking for hints.

Looking for hints.