Re: oil change question for force 85hp.
The bottom screw that you show is the forward/reverse shift linkage pivot pin. If you take it out, there's a good chance that you will have to take the lower unit apart to get it back in. Do not remove it.
The drain hole is located directly below the shift linkage pivot pin on the underside of the lower unit. Usually a small plug that requires an allen wrench to remove. After you remove the plug, remove both the "Fill" and "Vent" plugs and allow the oil to drain. May take quite a long time (hours) to drain completely. Once drained, re-install the drain plug and start filling up with good quality marine lower end gear oil using the "Fill" port. Keep filling until you see oil coming out of the "Vent" port. Stop filling and wait about 30 minutes and fill some more until you see oil coming out of the Vent port again. It's important to wait for the gear oil to drain all the way down so that you don't end up under-filling the lower end. Do this until oil starts coming out of the Vent port immediately when you start filling. Install the Vent and Fill plugs.
As a precaution, you should also change the two gaskets on the Vent and Fill plugs to prevent water leaking into the lower unit. The drain plug on the bottom is a pipe thread, so no gasket is required.
