Rebuild the shocks so they will perform equally in sets. I'd do this, especially being used, and not knowing what's in them. Checking for bent shafts as you go. Download the manual and start from stock settings. The thicker the oil, the slower the piston will move. It's always a give/take issue. Too thick of oil, and the suspension can't react to bumps fast enough. Too thin of shock oil, and your RC will bottom out after a good size jump. Tune the shocks to the terrain, and driving style. If the MTA-4 is anything like Associated's Rival, then 40wt shock oil would be a good place to start.