I am not sure what the objective in adding a dedicated cooler to the turbo would be. For the turbo and the engine there is an optimum oil temperature that should be maintained. Too cold and too hot is a problem. I am not aware of the turbos on the MB's suffering from a short life span due to bearing failure. I would also think if there is a benefit in cooling the oil for the turbo it would also benefit the other bearing surfaces and therefore it would make sense to place an oil cooler on the entire system.

TimK

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EPA 609 & 608 HVAC Certified
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83' 300D Turbo - 300K + Miles
00' 1.9L TDI Beetle
85' 6.5L Turbo, GV-OD, GMC Suburban,
67' 2.5L Diesel NA - Land Rover Series IIA

Here's what you do if it's so cold your diesel fuel gels. Smear some on some toast and stay inside until it warms up outside.