This discussion is good and should remind everyone that oil selection has two criteria - the viscosity and the additives. They are independent characteristics.
Viscosity-wise, it makes no sense to chose a 20W50 over a 15W50. At the fully warmed up temperature, they are both going to behave like an SAE 50 and by definition of the SAE viscosity grade system, the engine wouldn't tell the difference. Only at lower temperatures would the 15W50 behave "thinner" than the 20W50.
For ZDDP levels (antiwear additive) the chart shows the 15W50 at 1200 ppm Phosphorus (P is the active component rather than Zinc) and the 0W40 is "only" 1000 ppm. Historically, 1000 ppm is still a healthy level. The formulation I prefer technically is the diesel truck 5W40, which happens to have 1100 ppm P. 1100 ppm is the level that oils always had "in the good old days."
I pose this thought to you: highly turbocharged, high piston temperatures, highly loaded engine, oil exposed to contact with blowby gasses, some fuel or partially burnt fuel dilution... could be a over the road tractor-trailer or a turbo Viper, right? Diesel oils are formulated for all-day high load, to capture/contain soot and sulfur, acids, etc that get into the oil, extremely long engine life, and high temperatures. The diesel oil would be a very good choice, too, and is easily found at Walmart, KMart, etc.
Oil pressure is not a straightforward indicator of oil protection unless the pressure is measured at the last leg of the oil galley (which it isn't.) A garden hose without a nozzle has maximum flow and low pressure. Put a nozzle on it and the flow rate goes down while pressure goes up. Close the nozzle and the pressure goes up more, but no flow. Same with thicker oil - more pressure measured at the outlet of the oil pump doesn't automatically mean more oil downstream.
Oil in a journal bearing is pulled into the minimum oil thickness location. As the rotating member spins, it builds a "wedge" of oil that it rides on. The pressure under this wedge is thousands of psi, dwarfing the tens of psi the oil pump puts out. The oil pump only needs to be a delivery pump, it is not going to float a crankshaft over the bearings.
That is why F1 engines can use ultra low viscosity oils (because they spin so fast and pull the water thin oil under the rotating part) but they cannot idle below a certain speed (because the crank will sink through the oil and make bearing contact.)
Back on topic. OEMs wouldn't designate the 5W40 oil because 1) has more ZDDP which they believe has a long term negative effect on catalysts, 2) minor fuel economy penalty, 3) more costly for factory fill because it has more additives in it than the 0W40. But again, I think it would be a very good choice.
Does this twin turbo set up have an oil temperature gauge?
In New Orleans, La area. Do I have to order from Mfg ? Any help appreciated.
