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Originally Posted by X25
That's the main guideline by Bob (Bobistheoilguy) as well. As such, 15-20 PSI at idle is more than enough. Keep in mind, you care about flow, not purely just the oil pressure. We use the oil pressure as an indicator for the flow that does the lubrication, not the other way around. The thinner oil will have less back pressure (and will flow faster as well), and it does not necessarily mean it's providing less protection.
The LSX engines are meant to run on oil with 10 cSt viscosity (the viscosity of 30 oil at operating temp; 212 degrees F). In an ideal world, we would have 10 cSt in every temp, and engine and the oil pump would be very happy.
When you use very thick oil, it creates so much back-pressure that the oil pump just bypasses the oil back to the sump at cold starts, significantly reducing the flow. Again, keep in mind, it is the oil flow that lubricates, and the pressure is indicator of flow, and a very false indicator at cold start. The bypass gate starts opening up at a very low 50 PSI on LS3s as far as I know (Camaro). As such, with very thick oil, and especially if you use it all the time, your engine will have a very accelerated wear in daily use, which is why these engines never come prefilled with that oil. GM recommends replacing the oil back after the event anyway.
Viscosity of 5W-30 at 302 degrees F is around 3.1, 0W-40 at 3.7 or so, and 15W-50 at 4.5 or so. None of these are close enough to 10 (what engine is designed for), so all will fall short. The important point here is that 3.1 viscosity at 4.5 viscosity are not light years apart; just 2 cSt. You can't just divide one with another, and say that it gives 50% better viscosity at 302 degrees F; it would be misleading. Remember, at 100 degrees F, the OEM oil will be around 50s cSt while the 15W-50 will be at 125 cSt(!!!!); way too thick, and the viscosity difference is 75!
15W-50 should never be used for street.
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