here is the response from api about cat's synthetic diesel engine oil.
Cat DEO 5W-40 is not licensed by API, and I'm not sure why. API's program is voluntary, so Cat may have some reason why it didn't include the 5W-40 on the license. That vis grade has been licensed in the past.
If you're using the oil in a Cat vehicle or engine, you will probably be OK as long as it is acceptable under your vehicle's owners manual recommendations. If the recommendations say to stick with licensed oil, you may want to check with your dealer before using the oil. Make sure your warranty won't be affected.
We always recommend you work within Original Equipment Manufacturer recommendations.
Kevin Ferrick
American Petroleum Institute
Engine Oil Licensing and Certification System
1220 L Street, NW
Washington, DC 20005
USA
Tel: 1 202 682