Does anyone know what the shelf life is of the average oil sold in one quart containers? I decided to switch to Castrol GTX after reading the UOAs for it here. It also happens to be on sale now at Advance for 50 cents off a quart. So I already bought two cases (savings - $12). If I buy more, I may not get to use some of it til next spring or summer because I will probably use Pennzoil during the winter. If I keep buying it (savings go up), will it loose some of it's usefullness if it sits in the garage til spring?