Oil specifications

[BnObob]

With this inherited German car, I have an inherited owners manual. Unfortunately, the "oil" section says the weight of oil, but to go to the MB dealer and have them put in a quality oil to Mercedes specifications. Sorry,
but "I" own this car; paid for; not shared by Mercedes anymore. So, I take care of it dutifully. But at the last oil change, I only got 2,500 miles in clean city driving before the oil turned dark.

What spec oil (SE SF SH, yaddy.....) should be used in the M103 engine??
And every 3K miles? I have found one brand of oil to perform less than good,
but is there a prefrence for Mercedes? Recommended brands? Or any at the store that meet minimum spec & grade? Are there any to avoid? Should any additive ever be used? If so, what? Detergent, or high mileage, which is better? I sort of think the synthetics are touted as better, but are they really, or is a good brand of natural blend as good?

There is so much difference in price (for the same weight), and most I believe (maybe wrongly) is product marketing/advertising on the bottles. True or not?

Thanks for any help!

B. (1989 300E M103 engine)