I've burned unused oil before at a quart per tankfull. A friend had a partial case of old Penzoil 5w30 that he wanted to get rid of, so I took it and put it to good use.
If you buy it at the store, I would stick with light weight, NON-synthetic oil. If you get it from other sources like I did, just make sure there's no moisture or crud in it (pour some into an old mayonnaise jar or something and look at it).
If the oil's good and you add it in relatively small amounts, you'll be just fine.
Mike