On a 120K mile 235, no need to worry about the zinc level in the oil. You can run any oil you like. Stick with a 10W40 or 10W30 and change it every 3000 miles or once a year.
The missing zinc is hurting the newer engines with way more valve spring pressure then you have. Plus, its the modern fast ramp high energy cams that are taking a beating along with the new start up (break in) engines. Once the cam and lifters are polished from the break in or from 120K miles, there is very little chance of cam damage.
Oils from the '60 & '70 had less zinc in them then the oils today even after it was reduced. Its the cam and valves spring makers who asked for it to be added due to the extreme pressures and ramp angles. I just read this in the newest edition of Hotrod & Restoration (
http://www.hotrodandrestoration.com/)
Something to think about,
A stock 1937 216 engine had 42 to 48 lbs of valve spring pressure with the valve closed and 94 to 100 lbs of pressure open. A stock 1954 235 has 52 to 58 lbs closed and 155 to 165 lbs open spring pressure.
A new set of Crower brand SBC race springs can have over 400 to 600 lbs of pressure when open!
There is a enough lead and zinc in your oil pan sludge to last your engine for life time.
Points information, (
http://chevy.oldcarmanualproject.com/chevyresto/54154.htm)
Joe
