Recently purchased '93 2.5 Turbo w/ 143K that was a one owner, dealer serviced auto and and more recently an '82 300D with 113K that also was a one owner vehicle also with complete service records. Immediately started using Delvac 1, 5W-40 on both cars right after purchase with no consumption and leak issues; in fact the garage floor has yet to be the recipient of so much as a drop.
Became a convert to this oil after researching this and other websites while I was the owner of a '99 Jetta TDI.
Claims that synthetic oil is "thinner" and hence, compromises viscosity at operating temperature is a myth; if anything I have more confidence in the oil maintaining the viscosity of a 40W oil at operating temperature than dino oil. Further, the 5W reflects the oil's ability to flow at cold temperature to critical components more readily than a dino oil's 15W counterpart.
Summary: to each their own. I do suggest independent research be performed and conclusions drawn rather than deciding on which oil type to use from this scribe's experience.

I've converted every car that I have owned up until these MBs successfully, without leaks. The 84 leaked when I bought it (haven't tried synthetic yet), the 83 just started leaking and is being quite irritating. Just dripped again yesterday, so I'm going to look at where it is leaking now. I'll probably end up changing all the seals at the filter housing. Regardless I hope no more problems in the future, and I'm going to try to go/stay with synthetics. I thought 5W-40 was a little to lite for the 617 motor.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k