I like to keep clean…so I always use detergent.
Detergent engine oil was in use during WW2. In fact there was at least one article devoted to it in the WW2 Army Motors magazine. If vehicles were switched over from non-detergent, the article included instructions on how to proceed.
I have been using a 10w30 (instead of 10wt or 30wt mono-oils) in my jeep for about 20 or so years. They seem to work for me. If you want to run mono oils that should be fine as well but you need to watch your temperature ranges…it’s just easier with the multi-grade.
Using too high a grade of oil in your engine is not good for it. There are articles in WW2 Army Motors that discuss this. Higher grades then called for can cause increased wear. Hard to believe but as slippery as oil is, it still has friction. Friction leads to heat, etc, etc. Doesn’t mean that guys have used 20w50 (or whatever) for years, including myself, thinking that the wives tale of using it in a worn engine was the best choice was true.
I personally don’t think the additives do much for your engine but I don’t think generally speaking they will harm your engine. For more pearls of wisdom try reading Automotive Trouble Shooting for WW2 Wheeled Vehicles: Volume 1 and Automotive Trouble Shooting For WW2 Wheeled Vehicles, Volume 2. These two books could help you become a WW2 wheeled vehicle knowledge wizard.