I understand that you accidentally dropped 5 gallons of oil into your heating oil tank that already had approximately 20 gallons of oil at the bottom. You've taken the precaution of shutting off the furnace switch, which was a good idea. Here are some steps you can take next:
- Check the oil tank for any visible signs of damage, such as cracks or leaks.
- Check the fill pipe and vent pipe for any signs of oil leakage.
- Check the oil tank gauge and check the oil level to make sure it hasn't risen to the top of the tank and is in danger of overflowing.
- If everything looks okay, you can turn the furnace switch back on and monitor it for any signs of abnormal operation, such as strange noises or smells.
It's always a good idea to call in a professional if you're not confident in performing these checks or troubleshooting the issue, as there could be unseen problems that a trained expert would be able to identify and address.