I just did a google search for Autolite XP spark plugs. I clicked on the first site on the list. That took me to an Autolite page that described the XP spark plugs. Near the middle of that page I clicked on the prompt to look up the part number. I entered information for my car (98 Contour SVT) and found the listing.
It shows that either APP764 or XP104 were appropriate for my car.
I suspect that they are the same heat range.
Since they are intended to be a factory replacement fine wire with platinum side wire, I would not be afraid to use them as long as the price is reasonable. The biggest complaint I had with the Motorcraft fine wire is that they did not have a platinum side wire. Ford did a pretty good job of convincing me that dual platinum was necessary with the coil pack ignition. I will admit that the original plugs (Motorcraft fine wires) did not look badly worn when I pulled them out at 60,000 miles.
I have about 30,000 more miles to go before I need to worry about replacing the ones I have and maybe by then they will become common enough for the price to come down. Last I checked the XP was still much more expensive than the APP plug.
