Thanks. Sounds like more than I wanted to tackle. I remember having the fork oil changed on a previous 1500 with no fork removal. My reason for wanting to change the fork oil is due to time not mileage(6 years) and I was hoping that it could be done without removing the forks.
Here is a common sense answer for you. How old is the container of fork oil you buy at the store. ?? How long has it been waiting for you to come along?
Age is not a factor with fork oil. It is not like engine oil. Not exposed to contaminants , well just a few but not like engine oil.
The way it turns black and nasty is from the wear of the teflon off the bushings. The teflon mixes in the fork oils and turns it dark. I suppose a little condensation gets in it too especially from heat on the tubes during use.
There is no set service on the forks , so you just guess, put the bike on the center stand , lash the stand to the front crash guard and get the front wheel off the ground.
Wiggle it, pull it push it, it will have just a bit of play, if very much it needs new bushings.
Other than that to stay ahead of things I have just picked two numbers out of the air.
Riding two up, heavy and so on, service the forks at 35 K. One up , solo..........50K.
Then by noticing what is going on in there you can make the decision on when to service them next.
You can change the oil in the forks without removal. It is risky, but you can do it. Removal of the lower bolts will allow the oil to drain. They are not drain bolts per say, but the oil will drain. Thing is they are set to the tubes with thread lock. To the extreme. So trying to get the bolts out with normal tools , lying on the floor is risky. They will also get to turning and not come out, so now they are loose and still connected to the internal parts, so now you have to use a long screw driver or rod or something to hold the internal mech still. Hence the use of an air gun, it runs them out so quick it is done before it can play that game.
But if you do strip one, nothing says it has to come out right then, it can wait till you have it on the bench. The next time. But if it comes loose to the point it will not tighten back........hence the risk. Now no choice but to put it all on the bench. That is the risk.
But you can do it, and work the actuator up and down with the caps off, fill it up , use a little pump off of something with the pump line thorough a piece of wood or heavy cardboard and set the level of the fork oil, and put the caps back on.
Just risky to mess with the lower bolts at a bad angle. They are bad enough up where you can work on them.