Synthetic in newer cars is suggested for good reason. Usually they are higher reving engines and create more heat.
Anything prior to 1980, heavy weight diesel oil with all the zinc a flat tapped cam needs.
Its not really the oil you need to worry about, its the filter.
Regular engine oil and synthetic doesn't lose viscosity. But a bad filter will make your oil bad and full of contaminates.
Ditch the Fram filters immediately, they are trash.
Wix or Napa Gold....which are made by Wix, are FAR superior to any other brand. That's not just my opinion, its pretty standard industry wide.
But many shops are paid to use another brand, so you need to ask for Wix. Ive cut open 100s or more filters in my days. Fram are always goopy, and completely clogged. This allows particles to bypass the filter and grind into your internals.
Fram filters literally destroy engines.
The only time to go above 5W30 is if your engine has an oil leak, or you are driving a race car.
10w30 in very hot locations can help keep oil on engine parts while not running.
And, there is very little difference between very expensive oil, and very cheap oil.
Ive ran cars off store brand oils for decades.....so have my friends in the industry....from mechanics to collectors.
And "high mileage" oil is a scam.