After purchasing a revo about 2 months ago I immediately replaced the shock oil. When I push down on the rear of the revo one side rises faster then the other, if I do the same on the front it lifts in a level fashion. I ve removed the rear shocks and they don't look bent or anything. I'm planning to disassemble them & replace the oil. Can the oil in the shocks just be topped up or is it best to replace it. How often should the shock oil be replaced? Thanks