Originally Posted by Hotrod Lincoln
"Read my lips- - - - -straight 30 weight!" 5W-30 is water-thin and has no business within 100 yards of any stovebolt engine! What I'm recommending is plain old SAE 30 single viscosity oil. For the purposes of a flush like I'm recommending, buy the cheapest non-detergent oil on the shelf at the parts store- - - -it's only going to be in there 15 minutes. 5W-30 oil STAYS the thickness of 5 weight the whole time, and provides virtually NO cushioning effect between the parts in an old, loose-tolerance engine. You might as well take a sledgehammer to those fragile Babbit bearings!

Deal with the compression problems first- - - -a little goop in the cooling system is a minor concern right now.
Jerry
This is really good info. I recently changed from a 10-30 oil to a straight 30weight on my 235 because of a startup rod knock until oil pressure was established. the removal of the multi vis oil proved a huge benefit on this old engine.
Steve

1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
In the DITY Gallery
Video of the 261 running

1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration