Hi, I went through the company below.
http://www.yssusa.com/products_yss.html
They use nitrogen, the spring and rebound is manually adjusted. The rebound looks easy to get at but it looks like you will have to take the shock to adjust the spring pressure. I am setting it up for 2 up pulling a trailer. I did this with my easy to adjust oem shock.
I am told that the oem shocks leak from the o ring.
I may be missing something but if the air bladder is completely separate from the charge of oil, I don't see how adding oil to the air bladder will make it better, but again, I probably am missing something.
Good luck with this!
Tom