Thank you the lack of a owners manual with this car and a blank oil cap left me wondering appreciate the help.
Easy enough to view an owners manual on line by searching. Here is one found using Google in PDF form. Page 266 has the capacity and specifications of different items. Not sure but this may be a Canadian version but also has US information2001 Mercury Cougar Owners Manual
Here is a copy of the page with specifications calling for 5W-20 oil