The biggest jump in mileage will come with replacing the transmission and differential fluids with synthetic. With engine oil the biggest bang for a buck as they say is in colder climates. If you live in south Florida or south Texas where it never gets below 40 degrees there is almost no difference in gas mileage between conventional and synthetic motor oil.

10W30 conventional oil, with temperatures in the low 20's to high teens my oil pressure gauge would peg at 100 psi at start up.
With M1 10W30 Synthetic I have never seen over 75 psi at start up at the same temperatures.

Today's conventional oils are so much better then even 10 years ago.
The difference between synthetic and conventional motor oil has shrunk
to a minimal difference.

As far as chucks breaking off it will not happen. The synthetic oil will just dissolve the really bad stuff and that bad stuff fills up and clogs the oil filter.
If there isn't sludge under the cap on the valve train and camshaft there isn't much of any sludge on the internals.
I would still go with a good semi synthetic for the first oil change. I would also short change the semi synthetic at 3,000 miles and then fill with a full synthetic.