Diff Oil - Spool Question

just experiment with thicker oil till you find a viscosity that works for you

i have heard this works really well
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXNXJ0

I need a number to start, 50k or 500k?

500k is thick, but not thick enough to be close to spool action in a TRX 4 spider diff.
Silly putty or plumbers putty work well and will give you very stiff diff action but still allows some slip. It’s cheap to try and easily cleaned out if you decide you want something different.

Plumbers putty packed in diff cup.

Depends how tight you want it. If you want it really tight, like relatively hard to turn by hand, then you wont find an oil that will do that. 100k will give you good torque distribution, but is still loose enough to keep the rear end in check. Start from there and work up. If 500k still isn't enough for you, stick with the spool.

In the past, I've used actual car diff oil. SAE 90. When its confined to such a small space, it loses alot of its fluidity, and creates a tighter acting diff. It doesn't give the diff a locked/spool affect, it just makes it tighter under high diff speeds. Adding a little friction modifier, has yielded some good results too. This is only a personal preference though. I prefer this, because it makes the truck a little easier to put into a slide. If thats what you want...

yeh, I'm ordering some 500k and will go from there. if i want it thicker i might at that point put the 500k in front and thicker again in rear. will 500k in front be alright or will i loose some turning?

500k is quite thick, (the thickest available I believe), so its at the extreme end of the scale.
Putting something like that in a front diff will completely change the dynamic of the truck.
It will do 2 things. It will give you horrendous understeer in some circumstances, but it will also make the steering alot more direct other times.

It a complicated thing for me to explain at the moment, as I am several hours overdue for bed, but the best thing I can suggest, is try it and see if you like it. Pulling the diffs out is a long process, just for the sake of experimentation, but its a good learning curve. You'll be surprised at the difference it makes.

I went with 200k diff oil to be on the safe side, worked out great for me.