As a general rule, always use a multi-grade oil that has the narrowest span of viscosity appropriate for your temperature zone . In the winter, base your decision on the lowest temperature you will encounter, and in the summer, the highest temperature you expect. Remember that oil with the fewest additives are the best, which is why synthetic oils perform better at a wider range of temperatures. 10W-40 and 5W-30 conventional oils require a lot of polymers to function properly at such a wide range of temperatures, leaving these two oils prone to thermal breakdown. You'll see very few experts recommending these two viscosities anymore, and even though 20W-50 has the same viscosity spread (30), it starts with a thicker base (20W) and requires less viscosity index improvers (polymers) to do the job. The synthetic oils on the market today offer superior high temperature oxidation resistance, high film strength, very low tendency to form deposits, stable viscosity base and low temperature flow characteristics. All of the conventional oils are good enough for most engines, but synthetics are superior lubricants to traditional petroleum oils. Synthetics can be run for two or three times the mileage of petroleum-based conventional oils, and they do not react to combustion and combustion by-products to the same level as conventional oils do.
There are many benefits of synthetic oils that most people do not understand, and because of their higher price, are unlikely to buy. Below are a few examples of such benefits:
Friction Reduction: This is all oils' main purposes to exist. It means a reduction in engine wear, which helps them last longer and require fewer repairs.
So go full Synthetic: 5w 30 or 10w 40 I run on 5w 30 in a V8 7 series.
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Last edited by marti5; 10-07-2008 at 02:26 PM.