Well since a stock LST is around 5kgs and uses 30wt oil, I'd suggest 40 or 45wt to start with.
40wt oil in an LST shock is very thick. I was running 30-35wt oil in my 19lb buggy with the RC-Raven springs and it was plenty. More recently I've modded my pistons and will try out some weights, but you can read about that elsewhere.