I bet he didn't if he used 15w40. You can use a Diesel Engine oil in your banshee. A lot of guys run Rotella T 5w40 synthetic in 4 stroke atvs and motorcycles. The non synthetic's are 15w40. Diesel engine oil doesnt contain friction modifiers like regular car enine oil.
The hard shifting in to neutral is probably caused by the fact that 15w40 is a thick oil. You want to run a light oil to help improve shifting.
As for stop leaks, dont use them. If it is leaking that bad then just fix the problem right.