Short version:
Use a reputable off-the-shelf brand of engine oil. Avoid making the common error of thinking heavier-weight oil gives better protection—it does not. Using the lightest viscosity grade your engine's mechanical condition and your ambient temperatures will support will ensure quick lubrication of all parts at engine startup, thus reducing wear. For a Slant-6 engine in basically sound condition, 10W-30 is usually a fine choice. For a recently-built engine and/or use in cold weather, 5W-30. For a worn-out engine you're trying to squeeze a few last miles out of, 15W-40 or 20W-50 or straight 30- or 40-weight. Continue using good quality filters (Wix and Purolator are two good brands). When you get a chance, service the oil pressure relief valve to make sure it's doing its job, as described in these two threads:
thread 1,
thread 2. Avoid the temptation to put additives or "supplements" in your crankcase—at best they do nothing; at worst they do damage and harm, so just use unadulterated oil. Change the oil and filter at a reasonable and appropriate interval (3,000 miles is way too short; it's a foolish waste of money and resources). And…that's it! Pick one of the many other things that are much more worth worrying about. If you want to get educated and know WTF you're talking about on the subject of engine oil for old cars, read
this.
Longer, ranty version:
Science is wonderful, but it has its limits. For example, it stands absolutely zero chance when pitted against mythology handed down through generations from father to son and baked to impenetrable hardness by endless repetition as "common knowledge" in forums on the internet. Everybody knows your old engine will grind itself to death unless you add
this additive (no, wait,
that additive! No, you're both wrong, it's this
other additive!) to your oil, or always only ever use
this specially-formulated oil (no, no, not
that one, you gotta get
this expensive stuff from a special "dealer!") because the big bad government made the oil companies take out (no, wait, it was GM! No, it wasn't, it was the API! You're both wrong, it was Honda!) the zinc (no, dummy, it's the phosphorus! Shut up, you're both wrong again, it's ZDDP!).
Never mind that zinc antiwear additives are still present in current regular motor oil, but yesterday's very high levels have been functionally replaced by other chemistry that does at least as good a job.
Never mind that the current API engine oil tests really, truly, actually do include flat-tappet engines (two different ones!).
Never mind that the sudden surge in tappet failures within recent memory is directly attributable to the sudden and near-complete outsourcing to China of a giant swath of aftermarket lifters.
None of that matters, because everyone with an internet connection is a qualified tribologist and petrochemist.
In the vehicle lighting technical standards and specification world where I live professionally, we have our list of myths and half-understood bits of information that we roll our eyes about people swearing it's gospel truth right out of the Bible.
The oil people have the same tribulation: the general public reads a well-intentioned but out-of-date, incomplete, and oversimplified article, think they've understood that they have to use a high-ZDDP (zinc/phosphorus) oil in their old car, and then go shouting it all over the internet, whereupon someone else goes "Hey, yeah, I read the same thing!" and the myth gets amplified.
The evolution of engine oil antiwear chemistry did not start or end with Zinc, Phosphorus, or ZDDP. Don't worry/be happy about it unless worrying makes you happy, in which case don't be happy…worry!
Some people get a warm fuzzy by paying $10 for a quart of fancy-brand oil and pouring it into a car that doesn't need it…fine, if that makes their socks roll up and down; life is short and people should do what makes them happy.
For vastly most of us, there is no sound reason, need, or benefit to seeking and paying for specialty boutique oils. The lowest-performing oil on the shelf today is light-years ahead of the best-performing oil on the shelf when the Slant-6 was in production.
