My wife and I both drive Mercedes W123's with the OM617 turbodiesel. This is a 3.0L IDI 5-cylinder mill breathing through a Garrett T3 turbo that holds 2 gallons of oil and has an air>oil cooler. On all the enthusiast forums they preach nothing but 40 weight - but there are also a lot of people that get nasty when you mention synthetic. "The synthetic is too thin for these old iron motors. They weren't designed for it. Go by the owner's manual." Well... the manual says this: 32F and up: SAE 30/20w40/20w50 23F and up: 15w40/15w50 -4 to 50F: 10w/10w30 All climates: 10w40/10w50 Both engines get good fuel economy, use no negligible oil, and get Schaeffer's 9000 5w40 changed every 5,000 miles. I don't want to risk the motors on a casual experiment, but will using a synthetic 10w30 (to the chagrin of the old-hat enthusiast crowd) run any risk of compromising the motors through the cooler months here in KY? My gut says no, the modern 30 weight will be in better shape after twice the miles that a 40 weight would've been in the 1980's - but I won't discount the opinion of people with more experience than myself just on a whim.