Going by specs, 5W30 and 10W30 should be the same for a warm motor, but it ain't necessarily so. The stuff you get when you buy oil is simply not that perfect. For example, a lot of diesel motors run straight 30 weight and it is thicker than 10W30 even when the motor is warm. Also, most motors lose oil pressure at idle over time because of wear to the big bearings along the crankshaft.

My preference is for whatever the manufacturer recommends for a motor in good condition, but for myself, there is no way I would run 5W30 in an older motor that is losing pressure. Nothing less then 10 for the first number, and maybe 20 if the motor is seriously wore. Optimal lube is no longer an option when the thing is leaking the oil away faster then the pump is pushing it in.

.....and the facts/data to support this please.

As Mr. tjwalker's sig says...."without data, all u have is opinion". Just asking for data/facts, always like to learn something new (that's factual).

Woah there. Hold up. WHICH diesel engines? Volkswagen's 1.9L TDI likes Mobil1 5w40. Detroit Diesel's 12v71 likes Chevron Delo 100 SAE 40.

We're talking about the I6 here. Diesel engines have different requirements with regard to shear strength, contaminants, additives, sulphated ash, heat tolerance, and physical stability. In CERTAIN engines CERTAIN oils with lots of viscosity improvers added are problematic. There is such a thing as multigrade without VIIs.

That's odd. Can you please write Shell and have them fix this mistake?

Rotella T 10W30:
12.1 cSt @ 100C measured using ATSM D445

Rotella T1 SAE30:
11.9 cSt @ 100C measured using ATSM D445

Now there's some data....thanks Mr. salad.