It could be the oil suply line which goes across the push rods under the side cover. The line goes from a metering orifice in the block. As I understand it either the orifice and/ or the oil line can become clogged from build up.

https://www.stovebolt.com/techtips/engines/261%20rebuild/images/d-top-end-3a.jpg?crc=4177595665


Rock49 aka C-Rodg
work in progress
1956 Chevy 3100
261 engine