So I took my D in for an oil change to a local Lube shop and they recommended that I use 5w30 for my 5.9 engine. I had just gone with their recommendation in the past, but after reading these forums, decided I should be using 10w30 instead.

I have also been having overheating issues this summer, and am getting my mechanic to look at my cooling system this weekend (including switching to a 180 t-stat).

I did notice that the engine does not overheat as often or as quickly with the 10w30 as it did with the 5w30. This is the part I'm having trouble understanding, as I thought the second number (30 in both oils) was the oil's weight rating once the engine had warmed up.

Could one (or more) of you gurus kindly explain this to me? Sorry if it's a dumb question.

 

the oil weight should have no effect on the engines water temperature, however the 10w30 is the correct weight for your engine so stick with it.

 

Alrighty, thanks shrp!