I have a 1996 150 Johnson Ocean Runner.  It had been running fine on last years gas, until I recently filled my gas tank from almost empty and immediately had problems trying to throttle up.  My engine would bog down and eventually stall.  the problem was so immediate that I thought they put diesel in my tank!  I also eventually get a constant low oil alarm  but I also topped off the oil tank at the same time I got gas.  I found out I could pump the fuel primer continuously to keep it running so it didn't act like bad or contaminated fuel.  I changed the water separator filter and also the filter on the engine but still have the problem.  The engine starts fine and will idle ok at the dock and even run in gear at low rpm while docked but when I take the boat out it won't throttle up and then I get the alarm.