How much money do you want to spend on oil? The best stuff on the market, overall, is Mobil-1 Extended Life synthetic. It is also quite expensive. You can spend even more than that on premium synthetics like RedLine or pyramid-marketed junk like Scamsoil.
But, in terms of almost every performance and durability aspect, the plain old ordinary oil you can buy today is enormous leaps and bounds ahead of the best oil you could buy five years ago, let alone what could be had when the Slant-6 was new. So, you should feel no qualms about picking out a reputable brand of oil of the correct viscosity for your service, and using that. Valvoline, Castrol, and Pennzoil are good picks, IMO. I avoid the gimmicky "high mileage engine" oils, which tend to swell-up seals, temporarily reducing oil consumption but increasing it in the long run. Out of habit, I stay away from Quaker State, which nevertheless may no longer have the horrifying sludge problems it had some years ago. I also stay away from the 10w40-grade oil, which by nature is the most sludge-prone regardless of brand.
You mention your engine uses some oil. In that case, you may want to use a slightly heavier oil grade to help hold down the consumption. Several factors go into picking the best viscosity for your needs. How much oil does it burn, and how much does it leak, and what viscosity grade are you using now? Where do you live, and what's the weather like this time of year?
Some of the heavy-duty diesel-service oils are a good pick for some kinds of service. There seems to be consensus amongst oil geeks that Shell Rotella is junk, but Mobil Delvac or Chevron Delo are quite good. These oil geeks hang out on www.bobistheoilguy.com .
Filters: Pick a good brand. Wix (aka NAPA Gold), Purolator, Hastings are all solid picks. Fram is not. Use the big filter (Wix 51515, NAPA Gold 1515, Purolator L30001, Hastings LF-115) rather than the small one.
Additives that claim to improve or "treat" the oil, "treat" the engine, reduce friction, improve mileage, prevent dry starts, "restore" the engine, etc: No! None of them is beneficial, many of them are harmful, all of them are overpriced, and most of the claims made for them are baseless and fraudulent at best. There are some substances that can be added to the crankcase for special purposes (i.e., when the engine must be cleaned out for one specific reason or another), see for instance here, but that's a different matter. Unless you are cleaning out the engine, pour only engine oil into the crankcase.
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again, Wix really, really GOOD, and Fram really, really garbage