Possible to oil my Furnace Fan Motor myself and with 3-in-1 Motor Oil?
3-in-1 will work fine. Since I can't see exactly what your talking about in relation to your '2 orange rubber plugs' most of the time you do remove said plugs and oli until it's full.

Here is actually a video that better explains it in detail: https://youtu.be/UKoK_U_GKZI

Hope this helps!