Well these two folks have already stated everything I was going to mention. The cold start test is the best measure of compression without actually paying for a compression test. The 300D that I saw for sale which Wally (username: Hector111 ) has already purchased cranked up on Monday morning at 10:30am at 20F after sitting outside in very low winter temperatures for 72 hours. That was a good sign of excellent compression. I've been told that a diesel engine with good compression should be able to start without usage of a block heater down to about -20F.

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver