Some time ago now. I'm not sure whether it was driven by the military, or the motor industry, or just the oil industry getting its act together (given the reputational loss if one was rumoured to be making an incompatible oil).
My understanding is that all motor oils are either mineral, or semi synth, or fully synthetic. Synthetics are all group IV, polyalphaolefins, i.e hydrocarbons and therefore safely miscible with mineral oils. Having all the base oils compatible means it makes sense having compatible additives across the brands.
If you need higher performance, e.g. military jets, high performance compressors, you use Group V base oils which include phosphate esters and poly alkylene glycols. You *do* have to avoid mixing these with other commercial mineral oils. I'm not sure how much this is down to incompatibilities of the base oils or incompatibilities of the additives.