I like using the "sponge" when I have sex with my girlfriend, but with the sponge she dries up making penetration difficult. Could I use baby oil, or do I have to buy something like KY

Depends on your girls level of sensitivity "down there" I know that everything and anything that has a fragrance gives me infections. You should have her choose what she thinks is best for her or if you do buy something buy the KY.

Don't use baby oil, try some KY Jelly, they come in diff kinds.

Ky is always great and a much better substance than baby oil. Baby oil could cause a rash, give her a UTI and cause a lot of dryness AFTER your finished making love.

You may also try some extra foreplay. Try some oral sex on her, your saliva alone will make her much more moist, and her body will definitely enjoy it and create its own lubrication.

Grape seed oil is supposed to be quite good.
I am allergic to most lubricants so I have to buy a specialty one ($40 a tube) depends on your girlfriend really, what does she want to try?