Hi you might laugh but i use motor honey oil treatment in the rear diff.It works like a diff locker and don't bust axles like a spool,it has room to slip.I do this to all my 1/8 rc trucks and the truck with the diff in the center gets this too.And in the front i just use a lite oil.
I even carry baby oil in the tool box for shocks when out bashing.Me and my friends use this oil if we bust a oil seal or the top unscrews.And then fix it when you gets home,the baby oil might be too lite for the 1/5 trucks but not for the 1/8.The baby oil is easy to clean off you hands vs the silicone shock oils when out in the field.