I'm not one to attribute every mechanical malady to bad gas, but it sure sounds like you got a batch if it happened soon after a fill-up and both bikes have similar symptoms. The culprit could be water or low octane fuel (pinging might be what you're hearing on hard throttle) or something other than gasoline came along in your last fill-up. My guess is that your 'fuel' was diluted/contaminated with something cheaper. Since you're near sea level the bikes will want high-octane fuel to prevent pinging.
Your best bet is to empty the tanks and re-fill with the best available fuel. I'd do one bike at a time in case the local gas is worse, but you're in a large city, so good gas shouldn't be hard to find. I can't explain the oil in the air filter, unless it's been regurgitated from the crankcase by the poor running engines. I don't know the GS8 configuration that well, but my former KLRs and my DR-Z pass oil fumes into the airbox to be burned, and if something's amiss I've found oil in there.
Next, find all the auto parts stores you can and look for Seafoam. Their website doesn't list dealers in South America, but Pep Boys, Kragen/O'reilly, Ace Hardware, Checkers, CarQuest and Napa all carry it in the US. Get a couple cans for each bike and dose regularly to keep problems at bay.
HTH and good luck. Remember...it's all part of the adventure