OK, I have to open my big mouth again.
I don't think the county rep was talking about pouring a quart of 30 weight motor oil into an EB. They were talking about 'ONE" drop of maybe mineral oil which spread out over even a small container would not hurt anything except mosquito populations.
If the local government is spraying once a week in your area that's what you're getting 'ON' your plants, yard and garden, some type of oil or chemical.
I do agree it's easier to cover the water fill tube and the drain hole to keep mosquitos out (unless you're using the AWS and then it's just a little more work).
Oil will float on the 'top' of the water. The only place you can put oil in an EB is down the fill tube. There's no way the oil can get from there (floating on top of the water in the fill tube) into the actual reservoir of the EB. If you're using the AWS it's probably better to just cover the fill tube with some kind of netting or fabric.
We live in FL on a canal off a river. Do we have mosquitos? Oh yeah. We don't use dunks or bits and we don't have any issues with mosquitos in our EBs. The AWS seems to keep them out but during the bad times when we have lots of mosquitos we cover the fill tubes and drain holes. The new version EB is really easy to cover the drain hole with tape and a piece of screen. The original EB drain hole is a pain because the tape gets wet and comes off after a few weeks and has to be redone.
Am I going to put a drop of oil down the fill tubes on my EBs. NO. I think it would be really hard to get one drop to hit the water at the bottom of a tilted EB fill tube. Would I put oil in any other type of container? Sure, if the container has exposed standing water.
Just use common sense and do what you have to do to protect yourself from mosquitos and the diseases they can spread.DISCLAIMER: The last part of this post is is not for new EB users. This should only be used under extreme circumstances and should only be attempted by EBers with at least 100 years experience!
I have a better idea:
"I'll just duct tape the drain hole shut, fill the EB with water until it comes out the fill tube, duct tape the fill tube shut and don't have to ever worry about mosquitos again until I replant the EB."