Answer: If your engine was just rebuilt, I would use detergent oils. If your engine had not been rebuilt recently I would not recommend this. The problem with detergent oil arises when it is introduced to older engines that have accumulated sludge deposits over the years, or even decades. This sludge will often remain in your engine undisturbed-that is until the detergent oil begins to dissolve it. At this point it begins circulating with the oil where it can wreak havoc.
The question about the reduction of zinc additives in oil can only be answered through anecdotal evidence. The oil manufacturers’ claimed that the reduction in zinc should not have much of an adverse effect on our older engines. It would seem that they were correct. I have not personally heard of anyone whose engine was damaged by this reduction in zinc, and several engine rebuilders that I’ve spoken with have agreed.
Zinc is an additive in oils that is intended to provide protection to parts of the engine such as the camshaft and the lifters. It is harmful to catalytic converters, so beginning in 2004 the amount of zinc in engine oil was significantly reduced. We owners of classic cars represent a tiny fraction of the cars that are on the road, so naturally the longevity of our engines was not a major concern to the oil manufacturers. The American Petroleum Institute (API) has designated these oils "For all automotive engines currently in use," which would certainly include our classic cars. The API standards are so stringent that the military dropped their own standards early in the 1970s and adopted the API standards for oil products.