To buy the right oil you need to know SAE Viscosity Grade and API Quality Index oil for diesel or gasoline engine. You can select any Brand from the listed.
Example: for gasoline engines BMW 5 (E28) 1981, suitable multigrade mineral oil 15W-30 quality SJ, SH.
For bodies made in 1988, for the cold season, well suited mineral 5W-30 \ SJ, SH.
If possible, check the selected oil for compliance with the manufacturers specifications and service intervals.