Your motor is a 1991 model.
There is a grease fitting on the extreme lower portion of the swivel bracket (on the left side up against the boat transom side if you are standing behind the motor looking at the prop).
There are grease seals at both the top and bottom of the bracket with the theory being that you fill it up with grease (basically from top to bottom) and that stops it from seizing).
I would suspect like most motors of this vintage, that the last grease it saw was when it was packed at the factory.
I just went and double checked my thinking (own a 91 model myself) and there is no "tension" adjustment of any kind to tighten/loosen the amount of tension on the shaft itself - it "is what it is" and if yours is stiff it's because it's either seizing or full of crud (busted seal let cooties in etc).
I would suggest you attach a grease gun to the nipple (down at the bottom) and pump in whatever she will take - if a seal is blown you will see the grease oozing out of either the top or bottom of the bracket in short order - but that may get enough in there that you can work it and loosen things up.
Alternately, the bracket can be pulled and seals replaced/guts cleaned up, but that is a fair undertaking - best avoided if possible...