what diff oil would you recommend
 

I need to rebuild the front diff, I order the FML diff cup with spider gears, what wt oil would you recommend?

I was thinking I would use something not too thick.

What is it going in? Brand & model, and intended use?

e-maxx and I run it on the sand & sometimes dirt.

Emaxx diffs don't seal so the fluid leaks out. Use grease, plan to rebuild your diffs regularly, or upgrade to a sealed diff like the supermaxx, ofna, HB, etc.

I have the FLM cups, gears, etc. I am familiar. I'm also upgrading to Supermaxx 6-Spider diffs. I have them sitting on my bench.

what type of grease do you use? I will check out the Supermaxx 6-Spider diffs for next time.

Just remember, the lighter you go with the fluid (grease being the lightest), the more the diff will unload to the tire with the least amount of traction. For serious off roading in rough stuff, I like to go with something pretty thick.

Depends in part. If you want slower diff action use automotive wheel bearing grease. For moderate action use EP rated Grease that is lighter such as Mobil One synthetic. It has the same load carrying capacity but restricts diff action less.

Same effect as when you move up or down the diff oil weights.

Conventional Cheap Automotive Wheelbearing grease would probably do the trick. I used a synthetic version on the last buildup and it worked fine.

Pro, do you find that the grease lasts as long as the fluid, or do you have to change it more often?

When you have the right grease in there it lasts a long time. The issue is that the E/T-Maxx diffs are not sealed so using silicon diff fluid is useless since it leaks out. The REVO diffs are sealed enough to swap out the grease for diff fluids, and the ofna 6 & 8 spider or similar are also sealed.

As I'm sure you know once you have a dif that accepts fluids the fluid last longer and works better than the grease.