Smoke coming from exhaust after 15-20 mins of driving?

I have a 1976 ford ltd 7.5l. It drives fine but after maybe 15-20 minutes smoke starts to come from the exhaust pipes. I checked the oil and radiator and it looks fine. Oil and radiator fluid is not mixing.

Is there anything noteworthy about the color or smell of the smoke?

If you wipe the inside of the exhaust pipe with a white cloth, what do you get?

Maybe the engine is just worn out. (Short answer for a question short on details)

Perhaps the automatic transmission vacuum modulator is failing. This will suck transmission fluid into the intake manifold.

What COLOR is this smoke and how thick is it? Does iot leave a cloud that lingers in the air or does it dissipate easily even with no wind? Without knowing that we can't help you. Try again.

Number one is to check the PCV valve. It can create moisture or become clogged and then back pressure from oil can pressurize the oil in the crank case and push it into the emission systems. This is the cheapest and easiest thing to fix. Milky fluid means that moisture has built up and oil in the PCV system means that the valve is not functioning which pressurizes your oil system.
White steam (water) or dark smoke (oil). If it is just coming out of the tailpipe then this is something getting into the combustion chamber. Worst case is a crack water jacket in the block. If the rings were that bad then you would be talking about the loss of power also.
The likely candidate if not the PCV is the valve seats or valves themselves.

Steam comes out of every tailpipe for the first 5 minutes tops and it is gone. Sounds like you are burning oil. Stand in the cloud and take a sniff. It is a smell you will not forget. Burning oil. 1976? How many miles and was it all highway miles? If you drove in the bush fishing that is off road. So the car is geared lower. So number of miles is actually higher than what is on the odometer. It is a FORTY TWO YEAR OLD TRUCK. If it wasn't burning oil that would be strange. They do not last forever(only unless you only took it out on Sundays for 40 years… Then it is below the expected mileage for that vehicle.(or 420,000 miles is the expect average low end miles on an average use truck.

Get the compression checked.

What color is the smoke?
White?
Blue?
Black?

What color is the smoke?
Black-mixture way too rich.
Blue-burning oil
White-head gasket is going or gone.