first of all congratulations for the forum that I found through another forum under advice of a friend. I own an Alfa Romeo GT (turbo diesel with 170hp), i tuned the car bringing it to 200cv, now has a torque of 380nm at only 2200rpm (on a total of 4300rpm). I can use 5w30 with ACEA B5 (lower HTHS) or ACEA B3 (higher HTHS). So i know that B5 oil are thinner than B3 oil. I read many advices about the statement that thinner is better. But in a car with very high load at very low RPM, is better go to a 0/5w30 thinner b5 oil or 0/5w30 thiker b4? Is the statement "thinner that i can get and thick as necessary" still true? I see that this statement is always used about gasoline engine with normal/high rpm. But in a turbo charged diesel scenario (high load/low rpm)? I have read also a paper where in a high load and low rpm engine is better go to thicker oil (obviously still being among those specified by the manufacturer). Where is the truth? Remember that both specifications are allowed by manufacturer. I drive at medium speed in highway but i have always great torque at low rpm and often I drive hard with many short journeys. So which oil you choose? Thinner 5w30 (B5) or thicker 5w30 (B3)?