the heviest oil i could find was team losi 100 wt. oil.
it works better than the stock.
the way i refill my redcat shocks is that i:
disassemble the spring assembly by taking off the outside spring, to do this simply take the lower retainer off the bottom of the shock and let the spring fall off.
unscrew the top and then empty the shock bodies it sometimes helps to move the piston up and down to pump as much oil out as possible. make sure to keep the bladder in the shocks cap!
then i fill the shock body half way with shock oil. then making sure you don't over shoot the level of oil. move the shock piston up and down to release any air bubbles from under the piston let the bubbles escape the oil. then poring slowly to not get more air bubbles fill it to the top. when finished move the shock piston up slowly to release exes oil. then holding the piston at full compression (not extension) place the shock cap back on and while tightening the shock cap with bladder in it, release the piston and it should slowly move down. do not over tighten the shock cap as this will cut the sides of the bladder off and cause inconstancy and leaking.
thread me back and tell me how it goes!