is NOT what I would call "dreadful"
You're making the assumption that this oil genuinely meets the requirements of SL/A3/B3. I suspect if you put this ACTUAL 15W50 oil through the tests you need to meet these specs, it would fall woefully short.
The reality is that no oil formulator does any actual test work in an all mineral or semi synthetic 15W50 because (a) it's a tiny volume grade which no one cares about and (b) because it's a horrible, VII-laden oil which is a complete pain in the backside!
The way you 'play the game' to qualify a 15W50 is to exploit the Viscosity Grade Read Across (VGRA) rules. You might pass the Sequence IIIG oxidation/deposits test as a tight 15W40 which then you can read to the 15W50 position. You might pass a 10W40 on the VG sludge test and read it to cover 15W50. You might pass the IVA wear test as a 10W30 to read up. My favourite was always the Cat 1MPC diesel test which you could pass as an ultra tight 10W30 and legitimately read-up to cover the 15W50 CF position (a complete joke but there it is!). The idea is to always pass the 'easy' test to cover the 'hard' position.
The ACEA positions were even worse! I once got a squint at the Peugeot TU5 test results of the biggest European test house sorted by viscosity grade. The number of actual 15W50 A3 level TU5 test passes could be counted on the fingers of one hand of a man whose hand had once had an unfortunate accident with a food blender! (BTW one of those kosher passes was mine).
Most 15W50 TU5 'passes' are read-across from 10W40 results and that's where the real fun & games really start. You're not supposed to know what The Competition are doing but there are always competitor presentations which 'fall off the back of a camel' for one's edification. And you find out that someone has 'passed' the TU5 to A3 with a full mineral 10W40 with a Noack of exactly 13% & 22 SSI VII and you KNOW ABSOLUTELY that it's a big fat lie! However, as it's considered bad form to bad mouth The Opposition you can't say diddly squat.
15W50s are bad news. Avoid!