I use 15w40 in my diesels year-round with good results, also. However, I haven't owned a gas powered car with this much mileage before and was thinking it may need something a bit thicker to maintain good lubrication. The car uses about 1 quart every 2000 miles. Your probably right that I shouldn't mess with a good thing, though!

Thanks for the replies!

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‘84 300TD “Mountain Goat”
'62 300d Adenauer-350 Chevy - "Max"
'15 VW Passat TDI
'16 Dodge Ram 3500 - Cummins w/6-speed
'68 Mustang fastback
'55 T-bird
‘63 Studebaker Hawk