As best I can recall, in 1999 I would have been using motorcraft syn blend and a motorcraft FL400S filter on my 1989 thunderbird.
There's a chance I would have been using up some stashed Castrol Syntec-Blend, but I'm pretty sure I had discovered the motorcraft by that time.
Remember when motorcraft was still a good deal?
That car made it to 209,000 miles on 5k oci's before I stopped driving it.