ok, a friend of mines got a 92 accord 5speed (obviously) and his car was broken from august till like last month, so it sat in his driveway

now as far as we can remember his clutch didnt slip at all before it was broke

so im thinking that since its been sitting, the rear main seal rotted and leaked fluid on to the clutch, causing it to slip

is that plausible?

 

nope not plausible nor probable, myth busted !!
There is a big fat horny hairy honker flywheel between the clutch disk/pressure plate and the rear main seal.
But it is a great idea to change the rear main seal when doing a clutch job.