The shut of valve is located on the lower side of the oil filter casing and very easy to change and thats the first stop. If that does not fix it then move to the timing chain tensioner and oil pump issues, do a search on these two as many cars used the 104 engine, even S Class
If you have been using the wrong oil grade for a long time, then changing to the correct oil grade will not repair the damage. But it may give you better overall oil pressure.