I have been putting the conventional 5W30 oil in my 91' 190 E since I bought it in April and I have to top the oil up every two weeks or so. Oil alone is costing me tons of money. I read in the cars manual that 15W40 has a longer life span as far as having to frequently top it off. Any advice and suggestions??

 

If the climate is hot or the car has a lot of miles go withe the heavy wt. oil.

 

Use the recommended oil.
However, if you accept that your engine is using oil because it is coming to the end of its life (or at least requires major rebuild) using a heavier oil may give some improvement.
You could also try some of the "wonder additives" that are available from your local "speed shop" but don't expect that these will miraculously solve the problem. They may improve it somewhat however.
Engines unfortunately have a finite life.
Stuart