There is no change is viscousity, based on mileage. According to the ".......service products" list, 20W-50 is only good for temps. ranging 23 deg. and up. Since you live in Mass., I know it can get below that in the winter. 15W-50 would be good to as low as 5 deg. This of course conflicts with my TD's owmer's manual, which say 15W-50 is only good down to 23 deg. and the forementioned 20W-50 is only good down to 32 deg. Now, it can be argued until the cows come home that over the years since our manuals were printed, that oils have improved. The "service list" does not differentiate between synthetic and conventional oil, when it comes to recommended weights. Some also argue that a "thinner" synthetic is as good as a thicker conventional oil. For
Mass., I would go with 15W-50.
As far as Mobil 1 5W-30 goes, I don't think it ever was an approved oil. In Europe all engines take at least a xxW-40 oil, which has a classification of A3. Mobil 1 5W-30 is rated A1. The ONLY Mobil 1 sold here that meets this standard is 15W-50 and if you can find it 0W-40. According to the "list" of the 11 synthetic oils approved, only 2 are 5W-30, Castrol TXT Softec Plus and Shell Helix Ultra. They must be sold only in Europe. All the other oils listed are 0W-40, 5W-40, 5W-50, 10W-40 and 15W-50. Also, alot times dealerships put in whatever oil they get the best price on, like the dealer mentioned that put in Mobil 1 5W-30. You could challenge that dealer about their using a non-approved oil in your car.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
President of CT/Westchester Section