Which gear combination is right for me? I'm a 65-year-old male. And I have osteoarthritis in both my knee joints.

I've Fuji Absolute 1.9D bike and I use it every day for one hour for my morning exercise. 

I don’t use it to go places.


When riding on a level road, what is the best gear combination for me ?  


2:6 (front gear 2 and back gear 6) 

or 

3:8 


Why?
Will 3:8 combination, harm my knee joints?


As you know, 3:8 combination increases the tension and reduces the speed while 2:6 combination does the reverse. 

Thank you for your help.
Cycling is generally considered a great exercise for someone with knee problems or osteoarthritis in the knee joints because it doesn't jar the joint like some other activities. But like your question suggests there are some concerns. At a lower cadence (60-75) and higher gear inches your knee will flex fewer times, but at a higher level of stress on the joint and the muscles and tendons that surround it. Switching to the higher cadence (80-85 or higher) and lower gear inches you will decrease the stress on the joint, but significantly increase the number of repetitions. So, to minimize knee problems, all other things being equal:


Make sure that your fit and position is dialed in for an efficient pedal stroke with no bio-mechanical abnormalities.
Use the easier-to-pedal gear with a higher cadence.