Originally Posted by rxlmod
I have a 351 cleveland that's all rebuilt I broke it in using conventional oil. I was thinking about switching it to some good synthetic oil. But a long time ago I was told that the left over oil in the engine would cause problems when mixed with synthetic stuff. Is that true? If so how do I get it all out or am I stuck with conventional oil? Is conventional oil really worse than synthetic?
No, OMG you can buy synthetic/mineral blends from the major oil companies at the parts store.
Synthetics tolerate quite a bit more temperature before they fail. Their molecules are stronger making them harder to physically pull apart under load, the back side of this is it allows a lighter weight oil to be used while still maintaining adequate protection. This reduces power losses to pumping the oil and windage in the crankcase. They fail without memory, what that means is when synthetic molecules fail that failure is isolated to the molecules involved. When mineral oil fail there's like a memory association that causes molecules not involved in the failure event to spontaneously develop a similar failure. Because of this synthetics can be run for many more miles. Many fleets using very high quality filter systems don't change the oil at all, they just put in additive referesers as needed. But to this requires sending a sample thru a lab to see which additives and how much need to be replaced.
Mineral oil is cheaper!