Some wheel bearing grease to keep the o-ring in place won't hurt a thing -- I'm leary of RTV in case it hardens too much while you are installing, causing a small void somewhere or over crushing the o-ring.
Make SURE the slot for the o-ring and the face it seals on are completely clean and smooth -- doesn't take much of a rouch spot to leak.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!