I am looking at getting a new car for my Wifes job which requires alot of travel. I am tired of having a deignated oil for each car ! The truck is not much of a problem since it is used for vacation and towing a travel trailer.I have always used Castrol GTX in it with good luck and have recently thought about using another oil in it but now I am thinking why ? The Castrol has been good for it. I have a 2000 4.6 Mercury and am thinking about a new Honda Civic because I am tired of other problems with her Toyota. Would 5/30 Castrol work in both of these cars ? I see Honda wants owners to use that 5/20 oil. Could I use the Castrol 5/20 in the Mercury even though the owners manual calls for 5/30 ? If I used Castrol I could just have her take both cars to a oil change center or Walmart.