RepairPal Certified shops can diagnose and repair your 1994 Cadillac DeVille. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.
There is no information published on that drainplug, I'd get under the car with a wrench set and see what fits. The plug could have been replaced with another size in the last 16 years and nobody could tell you what size it is without checking..