Use the oil Mini says to use, which reads like is 0w-30.Contacted local mini centre to ask how much the genuine oil filter was and what grade of oil they use.
The workshop here always use 0W-30 oil in MINI's
I have always used 5w 30 long life in mine
Should I be using 0w-30 on a high mileage engine as I am thinking oil pressure may drop to be a thinner oil
Don't worry about high mileage.
If the engine is worn enough the oil pressure is low with 0w-30 oil you'll have to run way thicker oil than 0w-30. (The old used car trick was to fill the crankcase with gear oil.)
BTW, I put anywhere from 50K miles to 317K miles on various cars and not one engine had any issue with oil pressure. Or really any internal engine problems at all. But all got 5K miles oil/filter services.