Classic Cars and Modern Engine Oils – Myths & Facts

I’m sure you have heard the talk, seen some e-mails and read some articles or blogs about current engine oils and it’s use in your classic or pre-80’s vehicle. If not, it goes like this………..

ZDDP (zinc dialkyl dithio phosphate, which contains phosphorous, plus some zinc & manganese) has been reduced to levels in modern engine oils that will cause rapid premature wear, even failure, of flat-tappet camshafts. This also affects some distributor gears and non-roller cam chains. ZDDP, more commonly called ZDP (zinc dithiophosphate) by automotive engineers, has been used in engine oils for over 60 years; the main purpose being an anti-wear, anti-friction barrier between the lifter crown and the camshaft lobe. The phosphorous is poison to catalytic converters, so the new “Starburst/ API SM” engine oils have the ZDP levels reduced to eliminate early catalytic failure, which is “certain death” to non-roller cam motors. Use of ZDP additives or diesel engine oils, which have several times more ZDP and are perfectly compatible with non-emission controlled engines, are necessary for these older motors to survive. Running with the low ZDP levels in modern “starburst” oils is almost like running without oil in your beloved classic!!!

OK, boys and girls. Let’s put this BS where it belongs…..in your trash bin. Here is some sound engineering information from Bob Olree of the GM Powertrain Fuels and Lubricants Group, starting with a little history of ZDP use.

ZDP was first added to motor oils in the 1940’s as a copper/lead bearing corrosion inhibitor, at about a 0.03% range. Then in the mid-fifties, when the use of higher lift camshafts increased the potential for scuffing and wear, the phosphorous level contributed by ZDP was increased to a 0.08% range. This increased level was good for flat-tappet valve-trains, but at higher levels (above 0.14%) that showed better break-in scuff reduction, did increase long term wear. The level of ZDP was placed at around 0.10% through the 70’s, since it also was a good, inexpensive antioxidant that helped to prevent oil thickening in high-load engines. With the introduction of the catalytic converter, phosphorous was found to be a poison, reducing their effective life at the then-current levels. So as the catalytic life span has been slowly increased to as much as 120,000 miles, the current Starburst oils have had the ZDP level reduced to a maximum of 0.08%. Modern ashless antioxidants contain no phosphorous, so the catalysts survive fine at the 0.08% ZDP level. As for the modern “starburst” oils, all are backward compatible and all formulations for Starburst specification must pass 2 valve-train wear tests: one engine using an overhead cam with slider (not roller) fingers and the other engine using a flat-tappet cam system.

So, to conclude, modern Starburst oils contain about the same percentages of ZDP (.08%) as was used in the 1950’s to solve camshaft scuffing and wear issues. Their use will not destroy your classic flat-tappet motor. Additives and refining technologies have produced better oils with full backward compatibility with all older motors. As for new engine break-in procedures, many cam companies have engine oil additives to increase the ZDP levels for better break-in protection. Or you can use any of the lower viscosity (5W-30 or 10W-30) non-starburst diesel engine oils for new engine break-in. If it makes you feel better, run it all the time……myths are hard to kill with facts.

Reference: www.cadillacforums.com/forums

Butch Chaffer – May 2008

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