I found two hive beetles in my bottom tray and 6-7 small larvae crawling around. I checked my brood box found no hive beetles but placed a trap there anyway. Should I add oil or a hive beetle killer into the tray?
Hello and welcome to the Flow forum! 
Oil is good. Just fill it 30% full. I prefer pharmaceutical grade Mineral Oil (available from any pharmacy) as it doesn’t go rancid. When olive oil is used and is slightly rancid from oleic acid, it is repulsive to bees. They don’t need that inside their home! 
Hi & welcome to the forum.
My suggestion is different to @Dawn_SD, I suggest to keep it clean, dry & check around once a week, & no longer than a fortnight. My reason being that rain makes it’s way into the trays. Also honey can fall into the tray while harvesting.
If the tray is kept clean, with weekly - fortnightly checks, we quickly see if something serious is going on inside the hive, such as what @Paul_Stender found in this photo.
PS, this next photo is from a tray that never got checked in about 12 months from memory.
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I am a firm believer in oil but you have to keep a good eye on it, especially after rain.
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I use vegetable oil so it catches any beetles that fall in. If dry, they’ll get away to breed another day.
I only use a small amount just to cover the bottom of the tray and add a little cider vinegar which attracts them.
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