Modern oils have had much of the zinc/phosphorus removed as it was adversely effecting modern catalytic convertors. This can cause serious issues in a rebuilt engine that is running a flat tappet cam, especially during the initial cam break-in and if there are heavier-than-stock valve spring pressures.
You didn't mention whether your later model engine had been rebuilt or it just needed to be refired. My experience has shown that if you are cranking a freshly rebuilt engine with a flat tappet cam, it is wise to use an oil with 1800 ppm additional zinc. Granted, these oils are a bit more expensive initially but are much cheaper than having to replace a cam with a flattened lobe. I normally purchase this oil from Summit Racing in either SAE 30,10w40 or 20w50 for $5.95 a quart. Contrary to popular belief the Rotella Diesel oil, which at one time had a higher zinc/phosphorus content, no longer has the higher content and I had issues with lifter noise.
If you're just going to fire up a used engine with a stock cam that has been broken in, I'd pull the valve cover and check out the rockers for sludge. If the rockers are clean and the oil on the dipstick isn't too nasty I'd pull the sparkplugs, spin the engine with the starter to build up a little oil pressure, put the plugs back in and crank up the engine and warm it up until I had oil to the rockers. Then I'd drain and replenish the oil with either off-the-shelf SAE 30 HD or 10w30 while if some light sludge exists I'd use 10w30 and plan on initially changing it and the filter at 1000 miles.
With the price of oil at nearly $3.00/qt locally, and with the reduced zinc/phosphorus contents, springing an additional $20.00 for an oil change is cheap compared to replacing a camshaft and lifters and cleaning out a bunch of metal from the sump. As for additives; Lucas sells a "break-in oil" suppliment at about $14.00/bottle and GM sells EOS (which used to be $7.00) for $22.00 a pint so the 1800ppm oil at $5.95 is pretty cheap insurance against wiping out a cam and doing additional damage to your engine. Hope this helps answer your question.
Dave
