Progressive instructions say to follow the service manual for your bike but to never fill the fork fluid at higher than 5.5" from the top with fork collapsed with springs out. Many people misinterpret the instructions and think that they are supposed to set level at 5.5" below the top and that will end up overfilling the GL1500 forks. On 1995-up, the manual says to set the level at 7.6" from the top collapsed. And just to add, I personally prefer 10W fork oil when I had mine. Progressive does recommend 15W but I found it to be too harsh. 10W seem to be just right.
I understand the 7.6 because Race Tech says with emulator in place. This could make up forge 2.4” difference.
Once the forks have enough oil to operate and not foam, any more oil changes the full stroke air pressure. This is in the Race Tech manual.
Too much oil and it will blow the seals.
Viscosity is the only adjustment for compression and rebound. All we have are fixed offices for the fluid to pass through both ways.
“Square” bumps like raised up expansion joints want to make the fork move up faster than it possibly an because the fork oil has to go through a fixed size hole. This is why I went with the emulators. They open up under high compression/flow to allow the fork to move instead of jarring the rider or deflecting in a different angle.
Now I have adjustable compression. Rebound is still adjusted with fork oil viscosity.
I actually tried the Gold Valve Fork Emulator setup for awhile. I had it professionally installed by the original Service Honda when they were still up and running in Hammond Indiana. It was probably 20 years ago when I had it done but just remember that I didnt like it. I dont remember why because it was so long ago. I ditched the emulator and had to replace the inner pipe as part of the installation called for drilling additional holes in it. I tried running with just the Gold Valve fork springs and had a serious pogo effect going on Once I switched to the Progressive Springs, I finally got the ride I wanted. I know the Gold Valve Fork Emulator setup had many positive reviews but like you mentioned, it's not for everyone.
The reason for my question regarding oil level is that I changed the seal and sliders on one leg about 200 miles ago. I then measured the level at 7.6. Now this damper is leaking. After over hauling, the damping was very hard with this amount of oil. Drained some oil, but it started to leak after a few days.
I know on mine, the 7.6" which is the specs in manual worked fine. Sometimes they will leak because of grime getting caught in the seals. There is a tool called the Sealmate that can clean between the fork and seals. Many times that will stop leaks. You can also make your own using a plastic soda bottle.
Also do you have the Schrader air valves on the fork caps? This makes it very easy to fine tune the oil level. Both sides must have the same level. But it is puzzling why the one you removed oil from is the only leaking. I would expect the other one to leak. I wonder if the seal wasnt installed correctly or another part was installed wrong or left out.
I use 10W oil. When I bought the bike, it recently had seals replaced at an authorized workshop. When changing wheel bearings, I drained the oil from the dampers. Here was only a small splash of oil, black. Therefore, I refilled new oil according to specifications. At this stage, there were no signs of leakage. Unfortunately, I pulled my right leg too far out, probably damaging the seal. But it took a few miles before the leak occurred. Do not have air valves on the dampers. I have a sealmate without success. New seals have been received from https://www.cmsnl.com/ so I guess it's just a matter of starting over.
There is a lot of discussion about the maximum oil level, but what happens if there is too little oil, is it a lower level?
Too little oil will not provide enough damping. Too much oil and the seals can blow. A little either way is fine but too little or too much, not good. And as far as fork oil level with Progressive Fork Springs, there really should be no discussion. The instructions are very clear to follow the information in the service manual. The service manual calls for 7.6" from the top, collapsed, and springs out for the 95-up so that's what it should be set at. Like I mentioned above, the confusion comes in because these are a generic set of instructions to cover many models. If you read slowly, the one section does mention 5.5" from the top. But paying attention, it says that if your service manual calls for a level higher than 5.5", only fill to 5.5" from top. Since GL1500 service manual states a lower amount, you then follow the service manual recommendations.
I am attaching a link to the Progressive Suspension Fork Springs Install instructions so you have it.