1989 yamaha waverunner. I purchased a 1989 25HP Yamaha Waverunner & was wondering how you check the engine oil?

Did you get any instructions from the previous owner? If it's a 4 stroke waverunner there will be a dipstick on the engine block, but I'm guessing it's probably a 2 stroke engine because most watercraft used 2 stroke motors clear through the 1990's. The oil will be mixed with the gasoline when you fill it, bottles of 2 stroke oil can be purchased at most stores that carry oil. Your waverunner can be mixed at 50:1, which equals out to be 2.6 oz of oil per gallon of gasoline. On a 2 stroke engine the engine gets lubrication from the oil mixed with fuel and will burn up fast if straight gasoline is used. Let me know if you have any troubles, but try to get ahold of the previous owner to get some more info.
Daniel

You don't, no need too because it doesn't take any.