Which Shock Oil Weight?
 

I have a Rustler build underway and I am not sure what weight to run. It is going to be a little heavier than stock because 90% of the parts are being replaced with aluminum parts. I want it to handle well but with all the added weight and not sure what weight oil to use. I was thinking 20wt but I am completely open to any advice.

If you are running the stock shocks, 30wt min. For aluminum traxxas then I would do 35wt front and 45wt rear. 20wt is really light for a traxxas as they are rather heavy, and adding aluminum to it does not help.
Bigger question is what are you playing on?
Nick

Mostly I would say large fields of dirt and grass, with some on road running aswell.

40wt front 35wt rear is almost always a good start!!
You will want to go softer front springs to keep
yourself from wheelieing in the grass

Don't you mean softer rear springs to keep from wheelying in the grass.