Fork oil. The difference will be the price of a liter in a motorcycle shop will be about the same as the price of a half liter in a bicycle shop.
Maxima makes good oil and it comes in 7.5 wt.. If you're 200+ lbs., you may want to go with 10 wt., which is easier to come by, as a lot of shops don't stock 7.5 wt. You can make your own 7.5 wt. by mixing equal parts of 10 wt. and 5 wt. OF THE SAME BRAND.