I called Advance Auto Parts to see if they still had Mobil 1 on sale for less than $4/quart. Unfortunately, it went back up again, but the guy told me Castol Syntec was on sale for $2/quart! Should I sell my soul to the devil and stock up? I realize it's not a true synthetic, but GTX is good oil and this should be better than that, right? What do you think?