I drive an aging 1986 Golf diesel to work, have it set on a timer to come on at 4am so it should be nice and warm at 6:15 when I start the car. The timer was set to kick on around 4:45am but with our current weather situation of below zero I pushed it back to four.

Having said that I have been known to leave a vehicle plugged in for the better part of a day on the weekend just in case I needed it to go somewhere, and that goes for the W140, which has 15w40 oil in it.

I think two to four hours is sufficient depending on outside temperature, any more than that might be waste but when it's this cold out I would prefer to err on the too long side.

my .02

steve a

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Steve A
92 W140 OM603
97 VW Jetta TDI
90 Passat variant TDI 6 speed MT
94 Chevy K1500 6.5TD
05 E320 CDI
+ others