I have some leftover oil (5W-30, 0W-30 altogether about 4qt) from previous oil changes. I just bought some Mobil 1 0W-20 because of a special deal at Advance Autoparts. Can I simply mix the 5W-30 and 0W-30 and top off with the 0W-20 if necessary? They are all Mobil 1 full synthetic. The...