Vehicle: 2004 Mini Cooper (1598cc Petrol, 3-door hatchback) (United Kingdom).
My knowledge-level: Low.
Background: My oil level is below 'minimum'. I suspect a leak and have booked it into the mechanics next week - the leak is not the concern of this question. In the meantime I need to top the oil up. The previous owner (I've had the car for 4 months) had been using PART-SYNTHETIC 10W/40 (I know this because they gave me their spare oil when I bought the car). My research today has found that the correct oil for the car is FULLY-SYNTHETIC 5W/30.
Question: Should I stick with the 'wrong' oil and top up with part-synthetic 10W/40, or should I add the correct oil (thus mixing two different oils)? Note that it is below minimum so it needs to be topped up with a decent amount.
(Edit: my question is slightly different to is-it-ok-to-mix-oil-of-different-viscosity-in-a-wankel-engine as I'm also asking about mixing partly-synthetic with fully-synthetic, not just different weights)