What will happen to a 3.5 v6 engine if a mechanic was using a 5w 30 oil instead of 10w 30 oil?

Hello, well, I'd say to use the proper oil. Then recheck for proper operation. Being that your particular area of California is often very hot, the additional protection needed from the "10" of the 10W30 grade is probably pretty important. I don't know about anything sticking; you could have internal engine wear. Tear down is required to find out the extent of your concern; I'd definitely lean on your "mechanic" about this. Note that I don't know anything else about your vehicle. Are you pushing like, 200,000 miles? Or is it like 50,000 miles? Things do wear out -- maybe not due to oil grade. Hard to know. A tear down will tell you more. www.repairpal.com/repair- shops -- if you'd like a third party opinion.