1990 535i: Which Oil? 10W-40 or 15W-50

[Ariel246]

I have always used 10W-40. But the last time I went for oil change, the new service men used 15W-50, as I was not paying attention.
The car has 190,000 miles and they suggested that I switched to a thicker oil.
Can I go back to 10W-40? If not, why?
Ariel

[sidewinder69]

I use 20W-50 in my 92 535I, BUT it only has 203000 KILOMETERS.

After that many miles you are going to have wear on the cylinder walls, it happens and it's normal. Since the tolerances in the engine between the piston rings and the cylinder walls aren't as tight as they once were from so much wear it is a good idea to go to a thicker oil.

Also you can go back if you wish, although in my manual for the outside temps that I'm running it suggested this type of oil so that's why I'm using it.

The other thing to consider is if you have an aluminum head you want a thicker oil to dissipate the heat better, no warpage means no headgasket failures. Which unfortunatley the M-30 engine is known for.