So I just bought a '91 300d, and the previous owners were quite meticulous about changing the oil every 3k miles. However, they always used Rotella T non-synthetic oil. I was going to switch to full synthetic on the next oil change, but while researching a few things, I came across this post in another forum:

"I reccomend you use whatever weight of oil that your manual indicates. The engineers at Mercedes intended these engines to use this oil, using synthetic substitutes on non-completly rebuilt engines usually causes more problems than anything. To be more specific, the synthetic oil acts like a solvent. Freeing years of built up oil deposits, deposits that prevent oil from leaking out of ancient gaskets. This more slippery oil, often slips its way past piston rings and past valve guides, past gaskets and makes what was a previously well running engine into a basket case."

Just curious what your take on it is. Has anyone had problems switching to full synthetic on older MB diesels? In my experience, I've never had any issues switching to synthetic oil, but previously it was always on gasoline vehicles with 30-80k miles on the clock... the situation may be different when dealing with a 20 year old diesel that has seen 180k miles.