Even when changing a synthetic formulation that contains refined petroleum more frequently, you have to deal with higher NOACK Volatility (evaporation/burn off). Refined formulations have a NOACK of 10-12%. True synthetic formulations are only around 5-8%.
MC 5W-50 = 12%
Mobil 1 5W-50 = 10%
Amsoil 10W-30 = 5.7%
Amsoil 10W-40 = 6.7%
The lower NOACK a lubricant has, the less oil that will end up vaporizing, then condensing in the catch can (if you have one), supercharger, the intercooler, intake manifold, on the intake valves, and in the combustion chamber. That's one oily mess I try to limit as much as possible.
If you check out the UOA thread, you will see that Motorcraft 5W-50 rapidly shears to a heavy 30/light 40 grade lubricant. Some sample have taken 2,000 miles to shear that much, others even less. The reason why I shun the use of MC 5W-50 that is recommended is because Ford knows it shears, they are OK with this fact...and still recommend using this lubricant up to 7,500 miles in the Shelby.
Why not choose a lubricant that will retain a 40 grade viscosity, shear less than 3% over a 4,000-5,000 mile change interval, has low NOACK @ 6%, a more robust additive package, is lighter during cold starts promoting less cold start up wear (MC 5W-50 actually specs as a 20W-50 above 32*F), and has performed better than any MC, Mobil 1, or Castrol 5W-50 sample.
Please keep in mind, this applies to Red Line, Royal Purple, OR Amsoil 10W-40....not Amsoil alone. The reason why I choose Amsoil is because I/anyone can buy it at dealer/wholesale cost. Royal Purple and Red Line would require me/anyone to pay store shelf or internet retail markup. In summary, this is about using a true synthetic base stock formulation in the 10W-40 grade, not just buying Amsoil.
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