When I compare "Fred's" automotive oil to CASTROL OR VALVOLINE, ETC..
same weight-they seem to be same quality with the SAE seal and same
API SERVICE SL &
SJ, But the price is almost .40 cent difference per quart. What is the
difference between name brand oils and the "Fred's" of the market? Am
I harming my cars (early 90's)by using the "Fred's" of the market?

As long as the oil meets the API service grade your car calls for you can use it. 
I personally use the cheapest synthetic oil I can find at the auto
store.  Any synthetic will outperform a conventional oil.
The difference in price is probably primarily marketing and container costs.