Reccomendation for Oil Viscosity
Oil viscosity is like octane. You only need what your engine demands. 20W50 or synthetic 15W50 is designed for a high performace engine that is constantly in the upper rpms. The only reason why I use 20W50 because in So. Cal it is 100F+ every day and I push my 300E sometimes. Too much viscosity is the enemy of fuel economy and cold startups. Also aluminum engine dissipate heat better thus they usually dont require as much viscosity. Iron blocks like M103 and M104 store heat thus thinning out an oil sooner. I think that some guys think the high performance label on 15W50 will actually make their 190E faster. My dealer reccomends a synthetic 5W30 for both my S-classes. So my reccomendation is for you to stick with 20W50 in the summer months unless you live in Alaska. But in the fall in winter try a 10w30 or a 5W30. Once you are running 20W50 does the oil pressure begin to decrease or is it always at 3. If it is always at 3 then you are running too much viscosity. Thinning out of oil to a point promotes the oil's seek and lubricate philosophy. Likewise, if the pressure is falling too quickly then you need to either 1. run more viscosity 2. perhaps synthetic 3. get an oil change and/or finally inspect the car's oil pump and sending unit.