There are lots of factors to consider. I run the lightest weight that I can for my local climate, while still maintaining adequate oil pressure. My manual says that .3 bar at idle is acceptable, although I have more pressure than that.
Here is my reasoning:
Heavier oil causes more drag on the rotating assembly, using more gas and robbing power.
Lighter oil flows through the bearings more quickly, carrying away heat and constantly providing cooler, freshly flitered oil to the bearings.
If the oil leak is bad and it bothers you, just fix the leak. I wouldn't play with viscosity to mask over a bad gasket.
BTW, I don't consider your car to be high miles. It's just getting started...
Experience is something you don't get until just after you need it.