These engines were design to normally operate above 2500 rpm. Most of us in the USA travel around at 1500, especially to and from work. This slow speed does not allow proper oil splashing into critical areas, and then the oil dries from the internal heat and forms sludge. Sludge build up is death not only for the engine but the seals as well. Too much sludge covvering the seal and the seal will dry up.
The mechanic was correct. Use synthetic oil. But also DRIVE your car, do not be afraid to put it in sport mode and down shift to low when going through residential areas. Especially when the speed limit is 50Kph or 30mph. My 1998 tranny will hold 2nd gear until I come to a complete stop, then I shift to 3 until the tranny gets to 2nd, and then shift back to Low.

Klaus
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1975 164 w/174,800mi (Sold) 1995 850T w/91,000mi, 1998 V70R w/129,000mi