Oh, and I am a fan of synthetic oils in certain situations, like transmissions and axles where the extended drain interval is an advantage, but in the crankcase of my miata I have tried all the synthetics and found that they sludge up just as fast as good dino oil. That car has hydraulic lifter issues that rival the 235- The holes in the lifters are too small and they clog up, and then 4 valves per cylinder clatter and sound terrible. I know when my oil is saturated with contaminates because the lifters start getting noisy. I have found that good old valvoline works just as well as mobil one, but the cheap oil only lasts half as long. I replace a quart of oil with a quart of ATF through the engine (has lots of detergent) for about 15 minutes before I change the oil and that cleans all the scum out. After I changed the timing belt at 145k miles the inside of the engine is still clean even though the oil is always black and ugly when I change it.
You can't use synthetic oil in some motorcycles that share transmission oil with the engine- the clutch won't engage properly. My bike has a dry sump engine with shared tranny oil and it is very picky about oil. Luckily we have dry clutches (well, unless our rear main seal goes Jeep) on our stovebolts.
