I always use the recommended Mobil 0W-40W synthetic oil in my SRT. I found an intetresting article stating the advantages of using thinner weight oil. Here is what was said: Thinner oil flows quicker at cold start-up to begin lubricating critical engine components much more quickly than thicker oil can. Most engine wear takes place during cold start-up before oil flow can reach all the components. So, quicker flowing thinner oil will help reduce start-up engine wear, which is actually reducing wear overall. • The more free flowing thinner oil at cold start-up, is also much less likely to cause the oil filter bypass to open up, compared to thicker oil. Of course if the bypass opened up, that would allow unfiltered oil to be pumped through the engine. The colder the ambient temperature, and the more rpm used when the engine is cold, the more important this becomes. • Thinner oil also flows more at normal operating temperatures. And oil FLOW is lubrication, but oil pressure is NOT lubrication. Oil pressure is only a measurement of resistance to flow. Running thicker oil just to up the oil pressure is the wrong thing to do, because that only reduces oil flow/lubrication. Oil pressure in and of itself, is NOT what we are after. • The more free flowing thinner oil will also drain back to the oil pan quicker than thicker oil. So, thinner oil can help maintain a higher oil level in the oil pan during operation, which keeps the oil pump pickup from possibly sucking air during braking and cornering.
I always thought that 0W-40 was actually 40 weight oil fortified to act like 0 weight when it gets cold. However, I was wrong. It's actually the opposite. 0 weight oil is fortified to act like 40 weight when it gets hot. It may not sound like much of a difference, but if you think about it, it makes sense. That's part of the reason when 0W-40 gets old and tired, it can't "act" like 40 weight any more. Viscosity improver (VII) molecules make it "act" thicker when it gets hot but wear out over time. Viscosity Improvers are polymer additives that reduce the impact of increasing temperature on the viscosity of lubricants. Consequently, VII additives extend the service temperature ranges under which lubricants can effectively operate. Here's how they work: Huge molecules coil up into balls when cold, thus having limited effect on the oil’s flow (viscosity). When hot, however, the molecules uncoil and stretch out, thus causing an increase in viscosity. So, the thin oil acts thicker when it gets hot.
