Strange but true... Please advise
 

Greetings all! I am the proud owner of a 1975 MC. It has the stock 350, but I have upgraded to a 4BBL. It's been in my family since 1983, and only has 58K on it (pic attached).
https://cimg8.ibsrv.net/gimg/monteca...7059c554b7.jpg
https://cimg2.ibsrv.net/gimg/monteca...ca96b2a405.jpg
My problem is this: When driving in town, staying under say 50 MPH, everything is as smooth as brand new. When I get her up to highway speeds, 65 MPH or better, the car begins to drink oil. There's no smoke out the tailpipe, and no drips on the ground. But twice now, only at highway speeds for at least 60 miles or so, the car has used every drop of oil in the crankcase.
Any information would be helpful.
Thanks!
Bill

Nice car, I always liked the 2nd gens. The 6th gen tried to bring back those classic lines. Post some photos of your engine and interior.

You didn't say - I assume you replaced the intake too? If I assume no other work at that age and mileage and you are not seeing any oil seepage on the back or underside, I would think it's going through the valve seals. Have you replaced your pcv valve? It's also possible that your rings are not sealing at continuous high rpm. Is your no smoke observation in your mirror or has someone followed you? Are you using synthetic oil? It burns clean - my 2001 goes through it a lot with no smoke and no leaking. I would recommend you switch to high-mileage oil to condition the seals as age is one of the usual issues too.