Is it a good idea to change a 1973 cad. from havoline 30 wt. oil to a full synthetic oil. I bought this car new. It has 66,000 original miles. Driven less than 300 miles/year. Stored inside.
Stick with the 30wt. oil unless there are other problems you are trying to solve like difficult starting in cold weather. Then you may want to go with a multi viscosity oil like 10W-30 but stay with the regular oil rather than a full synthetic.