Haven't ridden my mtb in the last several years and recently had it serviced so I can go riding with some friends who are getting into the sport. Took it out today for just a quick stroll down the street and back, and was playing around with the travel adjustment knob on the fork.

Something seemed odd because I thought I remembered this adjustment always hitting a stop at the high and low points, and about the time this crossed my mind, a bolt falls from the bottom and dumps maybe a couple tablespoons worth of oil on the ground and all over my wheel and rotor.

I flipped the bike over and stuck the bolt back in place and tightened it up once I got home. I'm not sure if maybe this bolt/plug was removed during the service and never got tightened properly, but really just need to know what I need to do about replacing the oil that was dumped out and if the shock could have been damaged in any way from this. My ride home was only about a mile on smooth pavement.

Take it back to the shop, explain to them what happened, and have them fix it (for free). I am assuming you were messing with your preload dial. The bolt that fell out of the bottom was apparently not tightened properly. Messing with the preload should not make it come loose. The oil that drained out is what lubricates the sanctions of the fork and needs to be replaced. Your short ride home should not have caused any major damage.

I just hate to go up there crowing and saying they did something wrong on something they may not have even touched. My repair ticket doesn't say they did anything with the front or rear suspension other than airing them up, which I imagine they only did the rear as it was the only one that ever seemed to leak down over time. If it's as simple as just pouring x amount of oil into the shock then I'm comfortable doing it myself.

It is that simple, but with caveats. What brand and model fork? Air fork or coil-spring?

It's ideal if you can find and download and read the service manual for your fork. Chances are it's online.

It's worrisome that you turned a knob at the top and a bolt fell out the bottom. It would be prudent to get someone to look over the fork and maybe service it and make sure that it's bolted back up properly and that nothing else is amiss. Have you ever had the fork serviced? This might be a good time.

It's not going to be that simple, as you don't know how much is in there already, oil levels have to be a set measurement (and weight of oil), so you need to start with the fork empty of any oil.

Would be looking at the PDF instructions of your fork (all major makes will be on their manufactures websites) to see how much, for how to service instructions, check out Youtube, as someone will (probably) have posted a video of how to on there.

Front shocks are nice if you do a lot of trail riding. When mine failed I replaced it with a old fashioned steel fork and have never regretted it.