Oil is always a contentious issue.
The arguments against using synthetic oil in an old engine are:
- Synthetic oil is not compatible with the old gaskets, so the oil will leak from the engine.
- Synthetic oil contains detergents, they will loosen deposits in the engine and these deposits will block oil channels in the engine, leading to engine damage.
Arguments for using synthetic oil:
- Synthetic oil is a better-quality oil, it provides better protection against engine wear
- Synthetic oil is usually less viscous when cold, reduces friction, so you get better mileage.
- Leaks are caused by worn-out gaskets, not the synthetic oil
- If you are worried about deposits which may block oil channels, you should always be worried, even when you use mineral oil.
- Synthetic or mineral doesn't matter, it is all about the properties of oil.
I have used 5W-40 synthetic for a while (about 20,000 km), before it was semi-synthetic 10W-40 in my slightly younger 1980-200D. There were no problems.
MB specifies MB 229.1
for the W115, there are several 5W-40 synthetic oils which comply to that specification.
Don't forget to clean the main oil filter dan change the by-pass filter. Also change the oil in the Injection Pump and the air filter.