I used to use 20W50 oil as my default choice for my cars in the 80s and the early 90s as there weren't many to choice from in those days and thinner oils tend to cause all sorts of problems for older generation of engines. Even Mobil 1 was offered in a rather thick 15W50 grade in my country.

That was then, and in the past 5-10 years there had been a push towards thin synthetics to liberate more horsepower and fuel economy from modern engines.

I had Shell Helix 15W50 in my M104 engine while it was running in and at its 1st service at 15,000Km switched to Mobil 1 5W50 and I could feel it had less drag on the engine and the engine was more responsive (I've taken into account the fact I'm replacing old oil with fresh oil and an improvement was expected).

Recently I switched to Castrol Formula R (semi-synthetic) 5W30 at 90,600Km and the difference was even more noticeable! At room temp with the oil in its bottle you could feel the differnence; so thin it feels more like tomato juice in there. The car feel a lot more responsive right away, and oil pressure in the lifters builds up within a second from cold (20C).

It has so far done 7000Kms and it has not shown any signs of oil burning at all, with the high ambient temp ( up tp 38-40C ) lately it has coped very well considered its only a 5W30.

I will get the oil analysed and post the results in a few weeks.

I am intending to give the 0W40 Castrol R a try at the next oil change. It's a full synthetic, meets MB's 229.3 spec and a lot more $$$ than the Mobil 1 5W50 offered here. On the shelf you could feel it's so thin in its container it's almost like water.

In the past I thought if thin oil only makes a difference in small, high revving engines, but now I'm totally converted to thin oil on bigger and more powerful engines.