Marv,
I have used 300,000 wt diff oil for the front, did i mention it is THICK?
I got that one from a store in japan (rcchamp) only thing readable in the bottle is "mumeisha" so that must be the brand.
For the rear diff I'm using 3,000 wt

I also heard that for the front diff there is some sort of clay or putty that is really good and would achieve the same effect. It is used in the graphics/art industry as an eraser (you rub this in your drawings and it absorbs the graphite without leaving any residue). I can't recall the name right now but it is not pencil eraser; It is a special putty. From what i hear that would be the best to get a tight diff.

have you made the chassis mods to save the belts from getting shred by small pebbles? that is a good one too, specially for those running on non prepped surfaces such as parking lots. The new sprint 2 flux chassis has the required holes to get the pebbles out instead of getting them sandwiched between the belts and chassis. The holes are just below the diff boxes, the older sprint2 chassis did not come with it so it's advisable to put those in.

will try to contribute more tips as i recall them... My current sprint is on storage, dissasembled and awaiting a full rebuild. just haven't had the time for it...