i think your misunderstanding the purpose of oil and why more expensive oils are more expensive.
becuase its an 8v VTR used as a daily, it doesnt need some super posh synthetic racing oil, it was designed to run on cheap 10w/40 by citroen afterall. im a real 'user' of expensive oil, mobil 3000 ive always liked but only becuase i leave the car sat for a while then after warming it up give it some real hammer.
back in the 70s with poor engine designs engines would wear out, some more expensive oils were possibly worth while ive taken apart loads of old triumph lumps and xflows and found they have +60 rings in them and still have massive lips from wear, i stripped a VTS lump the other day with 125K on it and apart from a tiny coke ring there was NO LIP! absolutely amazing really. modern engines, (90s +) have such good oil pumps, filter systems, channel systems and such rigorous engine testing under so many conditions driven under normal circumstances they will never break really.
in my eyes, buy £50 oil and change it after 8K or £15 oil and change it after 5K... you can never change it to much, but after all, as much as i love them it is a a VTR and expensive oil probably constitutes 10% of the vehicles trade value
the saxo when it was new had some fairly cheap 10w/40 in it. The engine was designed