[BobBray]

This is the story: Zinc dithiophosphate (ZDDP or 'zinc') has been used in motor oil for many years. It protects against 'scuffing' wear, like the wear between a camshaft and flat-tappet lifter. ZDDP has been slowly phased out over the years because it was found to degrade catalytic converters over time, particularly in engines that consume a lot of oil. Since the EPA mandated long warranties for emission control components, automakers switched over to roller lifters and cam follows and pressured the oil companies to reduce ZDDP in motor oil. These days, it's pretty much gone from most motor oils. Exceptions are racing oils and oils for diesel engines.

The question now is are oils containing ZDDP necessary for older engines? The answer is yes and no. It seems that flat-tappet engines really don't need much if any ZDDP as long as they are well broken-in. However, scuff protection is very necessary in a flat-tappet engine with a new cam and lifters. Without adequate scuff protection, a new cam and lifters can wear rapidly and not properly 'break-in'.

What this all means to us is ZDDP is a good thing to have in the oil you use in your GMC V-6. I think is always better to play it safe with older engines, so I use Valvoline Racing 10W-30, which contains ZDDP, in my 351. My engine is well broken in, but why take a chance? V-6 cams are not so easy to find anymore. One thing to keep in mind, however: Diesel motor oil has been used a lot in flat tappet gasoline engines, but be advised! New diesels have catalytic converters, and the oil companies are stating to reduce the amount of ZDDP in diesel motor oils. I think a racing type oil is the way to go now.

As far as additives are considered, who knows? I have never used any.