Hi Andy, welcome to iboats!
Your motor was built in Belgium in 1986. It should have automatic oil blend (an oil bottle under the engine cowling) in which case you do not add oil to the fuel tank. If it does not have autoblend, the mixture in the tank is 50:1 (50 parts petrol to 1 part oil).
Try to avoid non genuine manuals. A genuine mercury manual is still available - part number 86134 4, just google that number and you will find a supplier.
big thanks andy