You can only use the engine oil supplied by the Ford Dealer. For some strange reasion any other oil even oil that is more expensive will not stay in the engine between services. Ford developed it for all their engines.
OP,
I've used Havoline in every Ford I've ever owned, so has my father. He's been driving and working on Fords since the 1950s. I've never had a problem with them losing more than 1/4 of a quart in 5000 miles and that was in a 15 yr old 4 cyl mustang that I drove as if it was a 5.0 till Dad took it away from me.
The 2003 Ford Focus runs on regular unleaded.
According to the 2002 Ford Focus Owner Guide : ( 5W-20 ) engine oil is recommended
Green
Motorcraft oil 5w20
no
5w/30
5w-20
We use Pennzoil 10W30
yes use Mobil1
5W-20 , according to the 2003 Ford Mustang Owner Guide
With oil/filter change use 4.5 quarts of SAE 5w-20 conventional or synthetic oil, your choice. Tighten the drain plug to 17 ft/lbs of torque.
We use Pennzoil 10W-30