STP oil treatments are an unspecified mix of mineral oils and 1 - 5% ZDDP (zinc alkyl dithiophosphate). ZDDP used to be one of the major anti-wear additives in engine oils and back in the days of pushrods and heavy valve trains needing higher spring rates it was a good thing. It's still a good thing for classics with engines that are prone to prematurely eating their camshafts/entire valve train but instead of pouring a bottle of snake oil into the engine along with an oil designed for modern engines a better way IMO is to just use the correct spec of oil... several companies make engine oils for classics that not only have more appropriate viscosities but also have the appropriate levels of ZDDP in them, Millers and Valvoline for example
More modern OHC engine designs with lighter valves and thus lower valve spring rates can get by just fine without it (in fact need different things from their oils) which is a good thing as it slowly poisons catayltic converters and so ZDDP levels in oil for more modern engines have been reduced. The M103 and M104 fall into this category. The bottom end of both engines is incredibly strong and snake oil won't help their few weaknesses (valve stem seals on the m103 at high mileages, the origional head gasket design on the m104). Just service it according to the schedule and use a decent oil of the correct spec- stuff like Fuchs Titan syn mc, Petronas Syntium 1000 or Shell Helix hx7 (all 10w-40 and meet MB 229.1 or 229.3) is good quality and sensibly priced. I usually feed the M104 in my s124 Fuchs, it's done a chunk over 200k miles and still one of the few cars i've owned that uses so little it doesn't need the oil topping up between yearly changes
Add me to the list of those that wouldn't touch slick 50 with a barge pole. I have used STP but not as an additive when changing oil, just as an assembly lube when freshening an old pushrod engine although with a new camshaft i'd use moly disulphide on the lobes, lifters etc
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