[BarryB]

With the engine off and tilted up, you should see the shift rod. It is a skinny rod sometimes painted the same color as the engine and is positioned vertically from the powerhead and runs to the lower unit. If you shift from N to F or R, you should see the rod turn. If the rod turns, that means the shift handle is working, however, it could be a bit out of adjustment, not quite making the shift.

The shift rod moves devices in the lower unit that eventually moves the clutch dog. The dog is the device you hear and feel when it engages the forward or reverse gear.

If it's not a simple adjustment in the shift mechanism, leave the work to a qualified marine mechanic.

BTW, Just changing the lower unit oil should not make a difference, it's just oil.

Hope this helps.

p.s. This is a good reason to purchase the service manual for your engines, even if you don't work on the motors.