so there are a few roads around here that aren't exactly smooth... and I get the expected bounce, but then the other day I was cruising on the highway in traffic and we had slowed to about 45 from up around 75 and you start really paying attention for idiots who are going to change lanes/cut you off/brakes etc... and I noticed that as we crossed a bridge I got weird secondary and tertiary rebound on the front forks as I crossed the gaps the bike's 13 years old, only 20k miles, but its lived here in Texas its whole life I'm guessing... I'm a decent wrench, but have never messed with forks before. Is that part of the 20k checkup? is there a way to "test" the fork oil levels/age/etc without tearing it apart? or is it something that should just be done period?
fork oil should be changed. the viscosity breaks down over time and work load. The wrenching really isn't hard at all. You can remove the wheel, drain the oil out the bottom, and replace with fresh oil pouring it down the top. But removing the wheel and draining the oil out the bottom is half the work of removing the whole fork tube - so you might consider spending another $25 on a fork seal kit and replacing them while you're at it. If they have never been replaced, you're on borrowed time. And they will fail when you're 500 miles from home and leak all over your brake pads.
5th gen also had .8kg progressive front springs from the factory. I'm 220lbs rider, running 1.0kg straight rate springs and LOVE them. To the best of my knowledge it still has OEM valving. Might be worth considering. Springs cost just over $100/pair