The folklore has it that the lower rails are bathed in oil and are good for more miles than the upper.
I have never pulled the cover off, but I would like understand why the oil pump needs to come out to get it off. In the parts book it shows a what appears to be a slide or tension rail for the oil pump chain, and the pump might need come off to replace that piece.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe