I have a 90 420sel with 110k. I am having the timing chain service as we speak. The MB mechanic is replacing the guide rails, tensioner, and checking the chain for stretch. He did not mention anything about the lower rails. Should I be concerned that he has not mentioned it? Should they always be replace along with the upper ones or is it sort of like brakes where you will replace 2 sets of fronts for every 1 set of rears, as so to speak? I would think being a MB mechanic he would have checked the lower rails. Do the lower ones generally last longer since more chance of oil being there than the top ones? Any advice I would appreciate.