Couple of problems here, this is fine if the snake is a new arrival but you haven't treated the mites in the eyes or heat pits. It is also advisable to treat the areas around the enclosure as mites travel quite quickly and can be out through the vents before you have seen them. The eggs can lay dormant for very long periods of time. I had plastic tub I with oil used in an experiment to see if the oil method worked, I was left with an oily snake and a plastic tub that couldnt be cleaned properly. I threw it into the shed. 8 months later on a hot day I was cleaning the shed and inside the tub were hundreds of newly hatched mites. the eggs had adhered to the tub with the oil.Mites can be introduced by birds, visting rodents and of course other reptiles. That they dont feed on mammals or birds is irrelevant as they can live long enough to travel on them.
One keeper recieved a shipment of geckos that were might free. but the container had eggs that had adhered to the carboard. Fortunately he found them just as they were hatching.
Ivermectin is a drench, not a spray. You can OD your animals easily with it. I was told of a better product on the weekend. I'll have to chase up the name.
This is a bit misleading. Snake mites are pretty much host-specific, and bird or rodent would have to come into contact with snake mites (Ophionyssus natricis) via an infested snake to be carrying them - they wouldn't pick them up from the environment. The only way your snakes will get mites is from contact with infested snakes, or if some vector (a human who has handled infested animals) brings them in. Not rodents or birds. It is highly unlikely, for the reasons Michael has already stated, that wild reptiles pose a risk.
Also, the life cycle of O. natricis is brief, with eggs hatching between 1 day in a warm environment, and 4 days at the cooler end of the spectrum. It would be highly unlikely (impossible) for snake mites to be hatching months later in an oily tub.
There are many misconceptions surrounding 'mites'. Not all mites are harmful - indeed, you can purchase predator mites to kill and eat snake mites in your collection, and these predator mites won't harm your reptiles. We need to be clear that the mite we are talking about, Ophionyssus natricis, is the species of concern, it is a specialist parasite, and is specific to reptiles. Other mite species from other animals will not harm your snakes. Ophionyssus natricis is fragile - it drowns easily, it dessicates at low humidity, and it has a relatively short lifespan - 14-19 days in most circumstances, including incubation of the eggs. All this info is available on the web, from people who know far more about them than I do.
Good quarantine, and limiting the access of those who may be carrying them on their clothes from recent contact to your collection, should be all you need to do to remain mite-free. They are not out there in the broader environment waiting for an opportunity to conquer your animals... they almost need to be invited in!
Jamie.