i have used soft scrub with bleach and i have used a degreaser they sell at stores like mills fleet farm. spray it on and rinse it off. the soft scrub take a elbow grease.
The gasoline on a cloth rag works well, just sayin' anytime your working with gasoline be careful, WD 40 is another good choice. I also found that if you have a good coat of wax or polish on that area, it cleans up a lot easier too.
Hey, if this warm weather hangs around for long we'll all be cleaning the sleds soon!
I've been riding Ski-Doo sleds with yellow belly-pans for years now. I use a product called Purple Cleaner. I believe it is a CarQuest brand and you can find it at most automotive centers. Spray it on and watch the black residue run off. 2 or 3 coats and rinse/wipe clean. They're as yellow as the day they came out of the crates.
I used Arctic Cat belly pan cleaner (actual product from cat) on my orange F7 and my red polaris, works great with no damage to anything. gently scrub with plastic brush and it looks like new
I have seen it at HD and Walgreens most. Similar to Simple Green but this stuff really seems to clean belly pans the best. Watch vinyl. It can take the silk screening and color off of seats.