I don't know what the lubrecation guys will tell you, but i have swapped viscosity, brands, mineral to
synthetic, back to mineral, single vis to multi vis, on the same motor with no ill effects. I have even mixed brands, weights, etc within the same
oil change. I usually break motors in on straight 30 non-detergent valvolene. Then after that case gets used up, I change to the 20-50 racing
oil. There may be reasons not to do this, but they haven't hurt any of my motors yet.