Reply to "Oils, cavitation and viscosity"

I've been using Amsoil & Bel Ray synthetic oils in my motorcycles since the 1980s with great success. I've used Mobil 1 synthetic oils in my cars since the early 1990s, also with great success. I'm not aware that synthetic oils resist cavitation any better however.

10W oils are good for use in a Cleveland set-up with standard type bearing clearances. As are 15W and 20W oils. Standard main bearing clearances for street performance being 0.0020” to 0.0025” and standard rod bearing clearances being 0.0025” to 0.0030”. The guys I know using 10W oil in their race engines use 10W30, not 10W40. The 10W viscosities may resist cavitation a little better than 15W or 20W, but enough to make a difference? I can't say. What the thinner viscosities will do a little better is make the sharp 90 degree turn off the main oil passage into the branches which feed the center 3 main bearings. Therefore they flow a little more oil to the rod bearings, and keep the bearings a little cooler.

FYI Ford's original recommendation for the 351C here in the states was 20W40. But 20W40 didn't stay on the shelves long, by the late 1970s it was replaced by 20W50. 20W50 was the standard for several decades. In the last decade I've seen 10W30 become more popular while 20W50 has fallen out of popularity.

You shouldn't use thinner oil (0W or 5W) unless the main & rod bearings clearances are set-up tighter and more precisely for the thinner oils. If the bearings are set-up tight, then consider synthetic oil a necessity, because the bearing temperatures run hotter. Also, if you're using the thinner oils (0W or 5W) don't use fully groove main bearings either, use 3/4 groove main bearings instead. Finally, since the 0W and 5W oils will tend to squirt more easily out all the unproductive clearances; tappet bore bushings become more of a necessity with those ultra-low viscosity oils.

-G