Don't think changing gear oil will do anything, the gears are not moving.
Double check the shifter alignment. I had a wore out shifter on my 300 that would get the shift handle stuck between the forks on the shift mechanism, it first showed up as difficult to put into first at a stop.
They say there are no such thing as a stupid question. They say there is always the exception that proves the rule. Don't be the exception.
