OIL FURNACE HELP

It sounds like you've tried to make sure the furnace gets air.

The problem is though, if your exhaust fan is doing its job (and it sounds like it is), then every opening or air leak connected to the garage has a draft going toward your exhaust fans.

Think of it like a hose with a bunch of different sized holes in it, leaking. Small ones leak less, but they still leak. It's just like that, but in reverse.. There's no way to get any of those air holes to flow zero even if they flow less than the big vent/fan or less than the big open windows with the filters. This is why the furnace needs to be absolutely isolated from any low-pressure area.

Sounds like a sealed fire rated door (they have seals) could be installed between the entry and the garage, making a mud room in effect, to help isolate it? Hard to suggest anything detailed without a floorplan or something, but you sound capable of figuring something out.