I know if there are any o-rings or rubber parts in that diff, petroleum based lubricants will make them swell. When we ran 1/10 and 1/8 scale I used a small dab of silicone spark plug wire lubricant. It's not like a real ring and pinion in your car that needs an oil bath. If the diffs are shimmed properly you shouldn't need but a MINIMAL amount of anything on them.