oil leak where transmission and oil pan meet

What is your question?

I use 10/30 oil in my truck and after the last oil change in November I noticed the parts store give me 10/40 instead. I didn't think it would be too much of a problem but after a few weeks my husband noticed a leak. We changed the oil again on 12-31 and I noticed there was a plastic piece where the transmission and oil pan meet that was loose and that seems to be where the leak is coming from. Now the tow/haul light has come on and won't go off. Any suggestions?