What kind of oil should you use in a 1987 Yamaha YZ250?

Umm a 1987 YZ250 was a 2 stroke not a 4 stroke, but you should go to any local Yamaha dealer and get the oils for your bike, and depending on the oil you want wether it is transmission oil, or fuel mix oil, if you have a old bike I would use Yamaha's reccomended oil which is yamalube if it is a newer bike I would suggest synthetics if your bike is brand new.

For the gears, use an 80 or 85 weight motorcycle gear oil. Honda sells a good one. On the '87 it tells you how much gear oil to use on the side of the case. For mix, its a 24:1 ratio. Go buy a Clymer repair book from your shop or online.