just got this 20 ft.deck boat 4cyl gm engine 120 to 155 hp omc lever control electric shift.i been told its a 1968 or older.someone put 80 to 90 wt. gear oil in the lower unit.i put omc premium electric oil back in it but still wont go forward.i do have current & connection in the wire that goes down into the foot of lower unit.reverse & neutral are fine.my next plan is to go into the foot & hope i can see what is the problem.hope i will be able to find the parts.i will probably tear up more than i fix.please let me know what else to check before screw up.your advice will be very much appreciated.THANKS FATS!