Thanks to everyone who responded, including those who sent me links to
treatises that would have made good engineering PhD theses. Here's what I
believe is
the general consensus.
ZDDP (Zinc d...d...phosphate) is an important additive for our engines since
it works with molybdenum additives to protect the wear surfaces on tappets and
cams in classic engines. However, it isn't needed in modern engines with fuel
injection, roller rockers, etc. and reduces the life of catalytic converters,
so the API is no longer allowing it to be added to oil for general passenger
car use. That means that any brand which is now API approved in the long run
will be harmful to a tappet engine.
However, Castrol GTX 20-50 and Valvoline VR1 20-50, both of which are readily
available still have the additive, though one respondent noted that he'd read
that the amount in Castrol has been reduced. In addition, good quality oils
for diesel engines, such as Chevron Delo, continue to include the additive
since they don't have catalytic converters and some wear issues similar to
tappet
engines.
Also, boutique oils such as Redline, Royal Purple, Torco, Amsoil, and
Penrite, which are used primarily by vintage racers, still have the additive in
their
non-synthetic oil and have the weight range we should be running in our
older engines -- though they're typically around $5 a quart. However, since you
only change your oil once a year, maybe the extra $30 a year in return for good
engine life isn't an unreasonable price.
Thanks for your input.
Cheers
gary
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