Good discussion going here. One other general thing to note about oil is you don't want to use fork-specific oil in a shock. Forks run much cooler than a shock does and the fork oil is not designed to take the heat. You can however use shock oil in a fork. Oils that are labeled as a fork/shock oil should be ok since they are formulated to handle the task. Changing oil weights can affect the damping, but remember if you put in a heavier oil for stiffer compression on bumps, your rebound is also going to be stiffer (slower) and the shock may pack up more and start to feel solid in repeated bumps.
Good synthetic shock oils cost anywhere from $12/quart on the low end to over $20/quart and will provide more consistent dampening over a longer interval than mineral based oil. Getting a shock serviced for $25 is a steal as long as it is being done correctly and with good oil. By the time you figure in oil and the disassembling, cleaning, setting the piston depth, bleeding, and re-assembly they are almost doing it for fun.
Another thing that stock shocks are notorious for is having air in the oil due to the fast production required to keep them low-cost. Just another reason to have them serviced. The front track shock on my 2014 had so much air in it that the oil looked like tomato soup (it had red oil in it). This makes for very poor dampening characteristics.