There is a marking compound made specifically for the purpose of checking gear contact patterns, I've seen it come in white or yellow colors. It works better than grease because it covers the gear teeth really well but is thin enough that only the actual areas contacting each other show up in it. I've compared grease and marking compound on the same ring and pinion job, the grease made it look like a good contact but the marking compound showed the actual mesh a lot better, and made it apparent more adjustment was needed. The trouble I've found with using grease is it usually wants to drag onto the contacting teeth, making the contact area look larger than it actually is. I think NAPA would carry the marking compound, if not you can get it from any business that specializes in automotive differential/drivetrain components, that's where the stuff I use comes from.
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