If you’re on a lake that allows motorboats, what do you do to clean/filter/purify/treat water? I’m not aware of any filtering or water treatment system that can remove oil/gas discharge from motorboats.

I don’t do anything different, just use my normal water filter. For as big as most of those motor lakes are I don’t consider it an issue. Especially considering you’re normally filtering water near your campsite and not next to a running motor so you should be good.

I usually am only passing through these lakes so I plan ahead and carry the water I need on the motor lake. I try not to drink on motor lakes, or entry point lakes, with some exceptions.

I doubt there is any discernable difference in water quality between BWCA lakes based solely on motor usage/non-usage. The biggest source of contaminants is biological and the biggest source of those contaminants is fecal.

If you're concerned about chemical contaminants then activated charcoal, in line with a filter designed for biological contaminants, is the only real option.

To add a great deal more information: I suggest that anyone interested in really understanding the capabilities and proper use of water filters for the backcountry watch the entire series on water safety by the youtuber GearSkeptic .

The following video is relevant to this thread:

Gearskeptic on Activated Carbon Filters

I always make an effort to avoid surface water in my dirty bag but especially on motorized lakes. My reasoning is that most contaminants, hydrocarbon or otherwise, are going to be floating on the surface.