I usually run mine for what I feel is long enough for the oil to get going. Then I take it easy. These diesel won't really warm up until you use them. I once let the car idle for 20 min in the cold of winter and still it didn't get much off the bottom peg of the temp gauge.

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1977 300D Lost coolant while someone else was driving
1983 300D Can't run without oil
1985 300SD (gone but not forgotten)
1990 300TE 4matic Sold
1991 Yamaha Venture
1975 Kawsaki 250 triple
1974 Honda 200CL
1951 8N Ford
2008 Wildfire 650C