I use Mobil 1 15w-50. I dont use any oil or have any leaks.
1) Get your engine fixed first
2) Check and change your gaskets before switching.
3) Use a engine cleaner first
4) Switch to Mobil 1 and enjoy
You said you used Mobil 1 over the winter once right? well did you have any problems then? If not, continue to use it.