Direct injection is also another potential problem because it allows the intake valves to get the dreaded carbon buildup. Probably one of the reasons they went to both direct injection and port injection on the new 6.7, LS 6 engine. Interesting that it's also a 409.

My '73 could get 25 MPG on trips when it was new and stock.

 

The fuels today are said to have additives and detergents with antioxidants to cut down on gums and varnish. It's sounds good but without port injection the intake and intake valves gunk up, CRC makes a intake valve and turbo cleaner that's made to clean the intake and valves. I have a can but I have not used it yet. GM makes something similar. I would like to use a Bore scope before and afterwards to see the difference. Maybe this summer after my daughter's house remodel. The PCV is the worst for intake Gunk. I have been thinking about using two oil catch cans, one for catching and containing the liquids and the second one remove as much oil vapor as possible before its returned to the intake manifold to cake the vapors to the valves, and still be epa compliant. Still just an idea. Using nonethanol fuel today you can achieve good mileage but just not as cleanly. I would like to play with the mpg this summer with the 77 vette, ethanol and non, looking for 20 mpg. (maybe)