Normal cars have manuals which say which oil type you should be using. Like 5W-30 for Toyotas etc. Some even differentiate between summer and winter and recommend thinner oil for the winter time, but not necessarily. Some cars even have the correct oil type written on the oil cap...

As to my 2001 ML 55 AMG, its manual only briefly says that I shall be using Mercedes-Benz "Spec 229.1 or 229.3" oil, and that is it. But what exact type?! Fully synthetic? Semi? Mineral? 10W-40? 5W-40? 5W-50? Smth else? For like 10 years, I have been feeding this car with fully synthetic 10W-40 oil (which matches the said MB spec), just for my convenience / for unification, because that is the oil I am using for my other car. But I am still curious, what the truly correct type (or types) would be for my very motor? Is there some technical bulletin or some reliable source which would say this explicitly?

P.S. Yes, I tried calling local dealers. Each one offered smth very different, and they all sounded like they just wanted to sell what they had on stock. And I did use Google with approximately the same results.

Thanks in advance for meaningful inputs.