Not on the Mini but my Minor Traveller - but it's relevant I think.
I fitted one of these for the oil pressure gauge off take
Neater than a T piece
It can also be used for a oil temp take off but is only suitable for earlier engines as later ones had a different length banjo bolt. I suppose you could drill and tap and existing banjo bolt but maybe it would weaken it too much.
https://www.ebay.co....tm/164633078441