I just use Kyosho Diff Gear Grease #3000 it's a bit more expensive at a tenner a pot but you don't need to mess around modifying the diff or anything just stick a load of it into the diff, put it together and forget about it. Because it's a grease and not an oil you don't need to worry about it leaking.
You can get it here
http://insidelineracing.co.uk/new-pr...e-3000-details