I own a 2013 chevy spark. It rear left shock is leaking oil gradually. Should I change it now or wait till it empties off? If it should be changed, should they be changed in pairs?
I saw a really minor oil leak in the shock of my new 2011 Toyota Yaris shortly after purchase. The shock worked perfectly, and nobody in annual inspections complained about a broken shock. I also didn't feel any reduced effectiveness of the leaking shock when driving. I just ignored the problem, and after a while the shock was so dirty that you couldn't see any visible oil. Yes, the very small amount of leaked oil was probably there, mixed with dirt.
Shocks are usually changed in pairs. If your vehicle is 4 years old, this advice is very sound for you, as a 4 years old shock can have reduced effectiveness. You don't want the rear left shock to be much better than the rear right shock.
So, my advice would be to ignore the problem if the leak is very minor. If, on the other hand, it is leaking substantial amounts of oil, replacement may be advised. Most parts of the world have mandatory inspections of cars, and they will surely tell you when the shock needs to be changed, so if you're on a tight budget, you can wait for their test results. However, more important than that is the feeling of the shocks when driving the car. If you feel the car is dangerous to drive on a bumpy 120 km/h highway, replace it now!
Any leakage means the shock is not 100%. However , at least thru 1980 year model most american car shocks leaked, so were not at 100 % but were driven anyway. My first new car ( 1979 Olds 88 ), I replaced the shocks before 20.000 miles. When I returned them to the dealer , he refunded me some cash, no questions asked. PS - I did not want any OEM shocks.