Your 1989 M103 engine can use many different oils but make sure you use the right viscosity. I would not put 20W-50 in there in NY. 15W-40 would be the highest viscosity I would put in that car. (Many of these M103's were filled with 15W-40 HDEO from the factory).

Your owner's manual will actually give you a chart showing oils something like 5W-30 and 10W-30 can be used for up to 86 deg F. Many many of these engines have already run to over 200,000 miles without engine troubles using mineral 10W-40 or 15W-40 oils with frequent oil changes. The valve seals and head gasket may fail but the engine in quite bullet proof in the lubrication department.

The 0W-40 and 5W-40 are nice because they can pretty much be used anywhere on the Earth. No need to select anything so it is kind of a intelligently dumb solution.

One thing though. DO NOT use ATF in the self-leveling suspension. It requires a special fluid that is clear in color. There are two types on the market you can use. One is mineral based (Febi) and another is synthetic (Fuchs). Your car calls for the mineral type but synthetic can also be used. You can buy the fluid from the dealer or many online parts places and they are not that cheap, especially at the dealers.

For the power steering (which actually shares the same pump with the self-leveling but two separate reservoirs), you could actually use DEX III ATF in there. It is in MB's Approved Products list. A dedicated power steering fluid works too. I have used the Vavoline semi-synthetic power steering fluid in my 300E's for over 10 years (I change about every 30k miles). Newer models require special power steering fluid and some are synthetic.

A DOT4 brake fluid is certainly OK but DOT4+ is certainly nice. Again, I have used the Valvoline brake fluid in al my cars but I change what is in the reservoir every 6 months or sooner. A couple of years ago, I was curious as how much fluid would diffuse or move from the reservoir to the caliper. I opened a caliper bleeding screw and the fluid coming out was as clear as what was in the reservoir. This car has not had a brake fluid flush for over 8 years but I always changed what was in the reservoir every 10 years. It appears there is far enough diffusion and movement to have the fluid mixed in the system. Remember MB specs these cars by keeping in mind that they need to bring a 130 MPH car to stop. That hardly happens in the USA if you obey the laws. A small bottle of the fluid (for about $2.50) is enough for two cars doing it my way.