At 8 years post-op it's highly unlikely that you'll be gaining ROM on your own, especially extension. It's very possible that you have some scar tissue (a common problem after surgery like meniscal transplant) and tissues surrounding your knee have tightened up (contracture)-- neither of which you are likely resolve on your own at this point. It's possible that PT would have helped some at the early stages of recovery from your surgery, but the longer scar tissue remains in the knee and the longer your motion is limited then the greater the chance that you have changes to the tissue structure. You want to avoid forceful PT, since it could result in damage to articular cartilage or other structures due to increased compressive forces from limited ROM.
Did you have any post-op followup with the doctor who did your surgery? At the time what did your OS say about your ROM limits?
Chances are you had limited ROM at some earlier point post op, such as within the first 6-12 months- post-op. If so, excess scar tissue is a very likely a culprit. Excess scar tissue is a condition called arthrofibrosis. Here's a link to a section called the Community Hub, you'll be able to find additional info from there, including: guided learning (tutorial written by one of the top experts on arthrofibrosis), literature review (interpretations of medical journal articles), and links to postings on the bulletin board. http://www.kneeguru.co.uk/KNEEnotes/node/632
If this link doesn't work-- Look for "Community Hub" in the upper right-hand corner of this page. Click on it. Then look for "Key notes" in the index along the right-hand side of the page-- click on Arthrofibrosis.
Probably time to make an appointment with a good OS, preferrably a knee specialist who has significant experience with scar tissue.