XO-5W50-QGT did not exist prior to the building of the Ford GT....
The 2005 Motorcraft Chemicals and Lubricants Catalog states:
SAE 5W-50 Full Synthetic Motor Oil, Part Number XO-5W50-QGT
Features:
• Motorcraft® SAE 5W-50 Full Synthetic Motor Oil is designed, engineered and recommended by Ford Motor Company for use in the Ford GT. It meets API Service Category SL/CF
• It is manufactured with high viscosity index, premium-quality synthetic base oils and performance additives to provide wear and oxidation protection under severe operating conditions. It is resistant to deposit formation, including supercharged and turbocharged applications. It provides quick flow to engine parts during start-up and lubricates effectively over a wide temperature range
• This oil is compatible with other synthetic and petroleum based formulations
Similarly, for the transaxle fluid:
SAE 75W-90 Premium Synthetic Transaxle Lubricant, Part Number XT-75W90-QGT
Features:
• Motorcraft® SAE 75W-90 Premium Synthetic Transaxle Lubricant is a premium-quality lubricant designed, engineered and recommended by Ford Motor Company for use in the Ford GT transaxle
• It provides excellent gear change and synchromesh performance even at low temperatures
• It provides complete lubrications for reduced wear
• This fluid is compatible with other synthetic and conventional mineral oil axle lubes
Not to say that other Motorcraft synthetics didn't exist, as they already had
SAE 10W-30 Premium Synthetic Blend Motor Oil, XO-10W30-QSP,
SAE 75W-140 Synthetic Rear Axle Lubricant, XY-75W140-QL and
SAE 75W-90 Synthetic Axle Lubricant, XY-75W90-QLS in their offerings of lubricants prior to and after the Ford GT development. In 2005, the only application for these two fluids was the Ford GT. In the 2006/2007 catalog, they changed the motor oil feature to read "...for use in the Ford GT and Shelby GT500. It meets API Service Category SM", but the transaxle fluid was still unique to the Ford GT. A notable difference in the change from API "SL/CF" to "SM" was a change in the color of the container cap from black to gold.