Modern engine oils in older engines: Addressing the additive issue
Zinc and Phosphorus-based extreme-pressure additives known generally as ZDDP have been present in engine oils for many years. Even in minuscule doses, these additives affect the hypersensitive exhaust catalysts used in current ultra-super-mega-low-emission vehicles. These extreme-pressure additives are also, so goes the claim, unnecessary in engines with roller-type valve lifters (i.e., virtually all of them today), which eliminate the sliding-friction junction of camshaft and solid non-roller lifter, which was what the ZDDP was used to lubricate/cushion. The latest API oil service rating for gasoline engines is "SM". The SM standard limits ZDDP to concentrations lower than optimal for our older engines, meaning these new oils give significantly poorer protection of the solid lifter-to-camshaft junction compared to previous spec oils. In everyday terms, the latest engine oils are not slippery enough when it comes to the camshaft-tappet junction.
Some of the heavy duty (diesel-rated) oils are good, but even many of these are moving to the SM service rating. It is only going to get more difficult with time to find SL-rated oils that contain the higher levels of ZDDP.
Generally, it's a poor idea to pour oil additives or "engine treatments" into your crankcase. Most of them do no good, and many of them can actually do quite a bit of damage. However, the low-ZDDP problem can be addressed successfully and at low cost if the additive is chosen carefully and used in thoughtful proportion. The ZDDP additives are available in the aftermarket. GM offers it as "EOS" (Engine Oil Supplement) under p/n 1052367 in the US, 992869 in Canada, and it seems adding some at each oil change would provide the extreme-pressure protection missing in SM oils without much of any chemical incompatibility risk, and without buying more costly exotic oils.
How much to add?
Well, the SM spec stipulates Zinc and Phosphorus content of 0.06% to 0.08% (600 to 800 ppm). GM EOS contains 5762ppm Zn, and 6221ppm P, or 0.6%. That means in a quart of EOS, you've got about 0.192 ounce of Zn and P. It's been well documented that a 0.10% to 0.12% concentration of Zn and P is optimal for flat-tappet engines, for example in SAE papers 770087, 831760, and 2004-01-2986.
To achieve 0.11% Zn and P in a 5-quart system, we would want 0.176 ounces of Zn and P. There's 0.192 ounce in a quart of EOS, but there's also 0.08% Zn and P in a quart of SM engine oil (actually, most of them run a little on the high side, as it seems, but we'll use the 800 figure). In 5 quarts of SM engine oil, therefore, you've got 0.128 ounce of Zn and P. Therefore, you're a little under 0.05 ounce short of what you want. Doing out the math and remembering that I've been rounding pessimistically, 8 ounces (1 cup, i.e., half a pint) of EOS would bring most any SM engine oil back within the optimal Zn and P content, which means that a quart of EOS would last you four oil changes. A quart of EOS lists for over $9, but can be had for $5.50 or so from a friendly parts department or GM Parts Direct. So, the added cost per oil change would be less than $2, even allowing for shipping charges.
So-called oil "stabilizers" (STP, Lucas, etc.) are mostly just motor snot, the kind that sleazy used-car dealers have been spooning into crankcases to quiet worn engines and temporarily stop exhaust smoke since your grandfather was in the market for his first car.
Most of the data for this article comes from the tables and charts here.