Elliot,

I hope someone with some specific experience pipes up soon, as I can only offer some pretty generic, simple advice on the question of replacing the O-ring. Replace it. These are usually cheap, subject to wear and aging. They may visually appear alright, but aging can change (reduce or eliminate) the elasticity of the ring, which will change its function, usually making it non-functional.

I am not aware of the condition of the inside of the booster, but if it is raw steel, without any paint, I would expect it would need a little oil coating as a corrosion inhibitor. If the inside is painted, there is likely no need for an oil film, but if the paint is not being attacked by the film, it is probably not detrimental.

Once again, I hope some other members pipe up with some useful information. Good luck, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)