All about oil
For years after the introduction of the high performance detergent oils, many mechanics continued to use non-detergent oils during the engine break-in period. Some mechanics still use non-detergent break-in oils with good success, although most would rather move in with their mother-in-law than treat an engine that way during those first critical moments. As you might expect, virtually no one uses synthetics during engine break-in.
Oddly, the reason why one oil may or may not work as a break-in lubricant has less to do with the base stock than with the additive package. Even many synthetics would be fine break-in lubricants if they were not so loaded with friction modifiers and extreme-pressure agents. This is not to say that you should never use it (as many racers already know), but that the additives may delay the break-in process. Of course, by using a non-detergent oil, you avoid the entire discussion of additives because there are so few additives to talk about.
If you want to play it safe, you are usually better off sticking with an SG- or SF-rated oil for break-in. If you want to try a non-detergent oil, the best bet is to leave it in the engine only long enough for the parts to scuff in — which can take anywhere from 20 minutes to a couple hundred miles — before changing to a detergent oil.
Even if your synthetic's additive package is mild enough for use during break-in, another good reason for using conventional oil instead is the cost of most synthetics. No break-in oil, no matter what it's made of, should be left in the engine longer than 1,000 miles. It is a shame to dump a bunch of costly synthetic with that low mileage.
The one clear exception we know of to all this is Synthoil, which should never be used during break-in. Many have tried it in many different types of engine, but it is simply too slippery. If you are a dedicated Synthoil user, you are going to have to use something else until your rings seat before you go back to using Synthoil.
(Webmaster note: Synthoil Corporation is no longer in business.)