If oil is leaking around the filter adaptor on 1991 Chevy 4 WD half ton pickup there a gasket or O ring that should be looked replaced?

Yes, there's almost certainly a gasket or O-ring (or possibly both) at the oil filter adapter on your 1991 Chevy 4WD half-ton pickup that needs replacing. The oil filter adapter is where the oil filter screws on, and a seal is crucial to prevent leaks. It's usually a rubber gasket, but some designs might use an O-ring instead, or even a combination.

You'll need to:

1. Identify the type of seal: Before purchasing a replacement, remove the oil filter and carefully inspect the adapter. Look for a crushed gasket, a damaged O-ring, or any signs of wear. Take a picture for reference at the parts store. This will help you get the correct replacement part.

2. Get the correct replacement: Bring the old gasket/O-ring (or a photo) to your local auto parts store. They can cross-reference it with your truck's year, make, and model to ensure you get the correct part.

3. Replace the seal: When you change the oil filter, be sure to also replace the seal(s) on the adapter. Thoroughly clean the mating surfaces on both the adapter and the new filter before installing the new seal and tightening the filter to the manufacturer's specifications. Don't overtighten; this can damage the adapter.

If the leak persists after replacing the gasket/O-ring, there might be another issue, such as a cracked adapter itself or a problem with the oil filter itself (though less likely). If that happens, further inspection and potentially repair or replacement of the oil filter adapter may be required.