Change it when you get home. As said before you aren't sure what the prior owner is running. Amsoil is the way to go. I have had many customers who have made the change from Mobile 1 and others to Amsoil (including myself). While Mobile1 is a good oil, and yes, you can get it cheap at Wal Mart. Mobile 1 has never been able to keep up with the quality of Amsoil engine oil. Amsoil was offering Guaranteed 25K mile oil changes in 1972, Mobile tried to do the same a little later and had to stop because their oil was not holding up. Here is how I see my maintenance. If the oil is designed to be able to be guaranteed for a 25K mile oil change, then the added cost to protect the engine in my $50K truck is a small price to pay for the best protection on the market.
When choosing your engine oil. Take a good hard look at the oil manufacturer recommended service intervals for the oils themselves, not the truck OEM recommendations. Once you have them in front of you, ask one question with the many others that will come up. If one oil is guaranteed to protect up to 25K miles, how much better will that oil protect than an oil that is not designed to last as long? Food for thought, but I like the confidence of knowing that I am protecting my engine with the best.
That being said, Amsoil also now offers different grades of their engine oils. Their Signature Series 5W20 is guaranteed for up to 25K miles or one year (the on year deisgnator is there so you don't just forget about it, Amsoil had no real shelf life limits), their XL Series oils offer a 10K mile or 6 month service interval and their OE Series oil is designed to be a more affordable quality synthetic for those that want to maintain their factory recommended service intervals according to the Ford Owners Manual. Take a look at them all, for those that are sticking with oil changes in the 3-5K mile mark and want to go cheaper, the OE Series oil is perfect and the cost isn't nearly as bad as most folks believe. Check the differences out below.
Signature Series 5w20
XL Series 5w20
OE Series 5w20
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