Re: 1966 c20
The rubber push on "OIL" cap was introduced in the early 70's mainly around 1973 depending on which GM division it was. GMC introduced the rubber cap in 1973 on the all their gas V6 engines. Looking at the pic of the 250 inline six it appears also to have the newer style valve cover as well so that could put it as a 1973-74, possibly someone may have just changed the valve cover but definitely not a 1975 on up as those engines had an integral cylinder head and intake manifold.
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