Low oil light came on and when I went to add oil I noticed rust under the fill cap. What could cause this?
Driving short distances can cause moisture to accumulate in the crank case where you would see white sludge like accumulation on the oil cap.
The worst case you're looking at could be a small coolant leak from a head gasket or intake manifold gasket where coolant can accumulate inside the crank case possibly giving you the rust look on the cap especially if the coolant is more watery mix.
As long as you are not losing any coolant change the oil every 3,000 miles or three months what ever comes up first to see if it clears up. You may want to clean up the oil cap or replace it with a new one so the rust won't fall into the engine.
Flush the cooling system and put in the correct mixture of coolant. Observe the coolant level for awhile to make sure you aren't loosing coolant.
If you need anymore help on this matter, get back to us and we will be glad to help.