From my own personal experience and what I've read, now at 1000 to 5000 miles after you've rebuilt the engine, is THE time to switch to the synthetic base oils if that is your intention.
Reasoning: You now have had the time to let the rings, bearings and valve system get "broken in" but no over done the tolerances. Make certain that you drain all of the old oil out of the engine, and use a HIGH QUALITY oil filter.
I did have one mechanic who professed that the proper way to do it was to do an oil change with the same weight oil, at a thousand miles. Then five hundred to another thousand miles later, (a week at the max), then do another oil change, this time to put in the synthetic oil. His reasoning was the first oil change was to get all the oil with the residue from the rebuild out of the engine and the second was to ensure that it was still going to hold up, and clean out any left over impurities. I could see his point, but synthetic oils and oil filtration media have improved exponentially in the last 20 years.
Mobil One is the lubricant of choice for the Corvette, as well as half a dozen other automobiles and trucks from the factory. There must be something good to it, for it to come from the factory in the crankcase.
