Stefan, don't worry about using 15/40 for the moment as the probable cause of the cam follower bleeding was a piece of dirt.
Change the oil and filter again in around 1,500 kms or in a few months time. Use 20/50 oil.
Have you removed the rocker covers and inspected the inside of your engine? If the engine has a high mileage or has not received regular oil changes, you may find a lot of black sludge inside.
