I have a 2008 Honda Accord four cylinder that is about to get it's first oil change. I bought 5 quarts of Castrol Syntec 5W-20 and after getting home and looking in my manual, the crankcase requires 5.6 quarts. I have 1 quart of Mobil1 5W-20 that is left over from the oil change I did on my wife's civic that also uses 5W-20. My question is can I mix the oils since they are both synthetic and the same weight, just different brands? I really don't want to go back out and buy 1 quart of oil.