Ferdman & Sixto... thanks for your input. I don't change the oil, so I may be wrong. I'll look again. (Of course, I could check the owners manual. ) When I left my dealer to use an indy, I gave him the last service invoice & told him to follow in their footsteps. I'm sure he did.
What would be long term effects on the engine of using that weight? I don't notice any performance problems.

Apparently none You probably have to run your engine pedal to the floor day in and day out for oil so close to spec to cause problems. Send a sample to Blackstone for analysis if you're concerned but I wouldn't bother.

Sixto
83 300SD
98 E320 wagon

The engine wiring harness and throttle body wiring are the greatest bugbear with the M104's.
However if changed the M104 becomes a much better long term proposition than the M103 with CIS. CIS has too many expensive parts (some N.L.A.) plus finicky distributor plugs and leads.

Head gasket on M104 is an issue but also on the M103 to a lesser extent.

I prefer the first gear start, better power and economy of the last series M104's.

Looked up one dealer oil change and they were using 10W30. It's appropriate viscosity for the climate in my area. Seldom goes much below zero nor above 95 in summer.


"In single grade oils, the lower the number, the better it flows at cold temperatures. Likewise, the higher the number, the thicker the oil. In regard to multigrade oils, the first number in the code, such as 10W in SAE 10W-30, means that the oil can still be pumped by the engine at a temperature as low as a single grade 10W oil. A 5W, like in SAE 5W-30, can be pumped at an even lower temperature, and a 0W, like in SAE 0W-30, will pump at the lowest tested temperatures. The W in the designation is commonly thought to represent weight, but in fact stands for Winter. The second number, 30, indicates how well the oil will flow when heated to 100C/212F or higher. "
Service Advisor - What does SAE 10W-30 actually mean? - Tomorrows Technician

Reading from the Owners Manual recommended oils chart: 10W30 is recommended where ambient temp in F is +50 to -4.

1988 124 260E 300E 300CE MANUAL.