1992 yz 80 replace front fork seals

SOURCE: front fork leaking oil from bottom of the seal.

This is one of the few things that I recommend you take to a shop - you need some very special tools and techniques to fix this problem. Sorry,
Kevin

SOURCE: front fork oil capacity

Hi
You can download the whole workshop service manual that has this info and heaps more from here. Hope this helps Thanks for using Fixya. 
regards robotek

SOURCE: 99 CBR 900 Front fork seals

If the seal has been cracked by the cold weather it should be replaced. For someone mechanicly inclined and with the proper tools this is not a hard job. For the novice it can be one of those times you could have sent it to the shop and spent less time and money, and gotten better results....
As to a seal conditioner, the seals today are generally made of synthetic material. No "conditioner" will work. If the seal is made of rubber you have a 90% chance that a product called 'MARVEL MYSTERY OIL' will solve the problem. This prouduct was created in 1917 and marketed in 1923. Google the product name for more hystory and uses today. Basicly, the oil causes rubber to swell. This tightens seals and thus fixes the problem. Most any good auto parts store has this stuff on the shelf. By the way, if the bike was purchased new, wouldn't this be a warranty repair?

SOURCE: Fork seal replacement

Did you try taking down the fork seal and cleaning out with wd/40 and a piece of 30mm film? You should always try this first. The seals rarely go bad. There's probably just a vary small rock holdeing it open and the film will dislodge it the wd is just to help things along.

SOURCE: front fork leaking oil from bottom of the seal.

change the oil seals and oil in the fork,and avoid wheelies on the bike and off road biking