when your not running your truck keep it off its wheels as this with weaken the springs causing it to sag. a thicker oil only makes for a slower rebound rate. its the springs that can either lift or lower the truck.since you have the adjustable alum shocks tighten down the little ring till it brings the back up to where you would like it to be or put the spacers from the stock ones on under the adjustable collar to help give it some lift.
if you got the shocks adjusted all the way and it still sags you need to change to a stiffer spring if its still not good... sounds like your running extra soft springs? red ones?