400E and 500E's got R134a in 1992.

All other models got it in 1993.

However, it is not a good idea to convert from R12 to R134a. The cars were not designed to use R134a, which doesn't cool as efficiently. The a/c will never blow nearly as cold, particularly at idle.

Besides, the R12 refrigerant is the cheapest part of your a/c system.

The longevity of a 2.6 is probably identical to that of a 3.0.

They will need the same maintenance and periodic rebuilds. Both will need a valve job / guides / seals between 225,000 to 250,000 miles. The engine will be good for another quarter of a million miles.

__________________
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".