The weight of the oil has no effect on how the truck will sit. The springs themselves do all the work in holding the truck off the ground. All the oil does is control how fast the suspension can work. It makes you believe that the oil holds the truck up but no matter how thick the shock oil the truck will still bottom out under the same pressure that was required to bottom it out before. If I were you I would go ahead and get some different springs to go along with the oil change. RCRaven makes a great set of progressive springs that run about 10-15 bucks shipped on eBay. As far as the oil goes the stock weight is something around 30 weight, but the stock shocks will work with anything from 25-45 or so. Some people run outside that range, but with a thicker oil the shock caps will start blowing off on harder landings. I am personally running the RCRaven springs and AE 35 weight oil all around with no preload spacers.