well the flux comes with the best diffs offered by HPI so I would say use those, and when landing off jumps and things let go of the throttle, much easier on the gears that way
theres 2 types of bulletproof diff bevel gears
you have the sintered and then the machined, basically the sintered diff bevel gear is made from powdered metal that has been heat pressed into shape but the heat does not exceed the metals melting point, where as the other type of bevel gear is machined on a cnc lathe from a billet of metal, which doesnt stress the metal as much
both are given the name of bulletproof, so the strongest gear out of the 2 is the machined gearing, but it comes at a higher cost, you can tell if you have 1 or the other bevel gear by the colour of it, sintered is silver and the machined is black, so when you purchase your new diff, id run it but if it ever failed then id replace the diff bevel gear with the machined gear, making it the strongest diff that hpi do for the savage
http://www.hpiracing.com/en/part/102692 thats the one i have, i know people have had no troubles over having sintered but i wanted the truck the best it could be so i put the machined diff bevel on front and rear diffs
so to sum it up, i took both diffs out the truck, stripped and cleaned them then assembled them but i replaced the diff bevel gear with the machined ones in the link above