Try this oil solution...
I had been using Sunoco 10-30w in my GN since I bought it new. I had won 6 cases of it at the track in the late 80's and just recently ran out of it. It met all the specs for the GN oil requirements.
So I did a lot of reading regarding the different oil solutions available for older flat tappet cam's that are available today. While there are some pricey products on the market that contain healthy doses of ZDDP I wanted something that would not eat up the cat converter and also get me through the tough New jersey emissions testing.
I settled on Valvoline VR-1 Racing Motor Oil (they make a street legal and also a track only version, so choose wisely) The race oil is available in 10-30w and 20-50w and a straight 30w, 40w,50w and 60w and is available in "dino" only.
Valvoline VR-1 contains 75% higher zinc than SM motor oils with a balanced additive package designed to work in both street-legal and racing applications. It also "contains longer lasting Zinc-Phosphorus additive that keeps higher levels of phosphorus in the motor oil where it protects the engine against wear instead of poisoning the catalytic converter and is the only brand of oil offering this unique additive across the entire line of oils including their SYNPOWER, the only synthetic offering this additive.
The Valvoline VR-1 Race Oil offers .13% Zinc and .12% Phosphorus while the Valvoline "not street legal" race oil has .14% Zinc and .13 % Phosphorus
For further info search Valvoline 's web site and go to product information ,race oil,and FAQ's .