slant 6 oil weight

valiantwagonguy said
I run Valvoline VR1 10W30 in my 170 . In summer I thicken it up with 2 qts of 20/50 same brand and also VR1 Racing . It has ZINK ! You need Zink for those solid lifters.

ZDDP means Zinc DialkylDithioPhosphate. It is actually the Phosphorus component of ZDDP that provides the antiwear protection and more is not better. Phosphorus levels over 1400 ppm can lead to increased engine wear and levels over 2000 ppm can lead to camshaft spalling. For flat-tappet engines, the Corvair Oil Guide recommends using Heavy Duty Engine Oils rather than racing oils.
See Valvoline Racing Oil FAQ #2 and GM TechLink - Engine Oil Myths

pt70 said
Assuming your car does not have a cat converter, I'd use SAE spec SG or SH motor oil, it has more zinc in it. SN has very little. Try looking at the labels on 'high mileage' oils.

As stated, 10w30 in the winter; 20w50 in the summer.

You'll have to comb garage sales to find obsolete oils like API SG or SH. Modern oils have to meet much more stringent performance requirements in addition to meeting the same specs as those ancient oils. The only difference is that STARBURST (and resource conserving) oils have a 800 ppm maximum limit on phosphorus. Heavy Duty Engine Oils have a 1200 ppm maximum limit on phosphorus and the Ford WSS-M2C171-F1 specification requires 1000-1200 ppm of phosphorus. See the API Motor Oil Guide:

For automotive gasoline engines, the latest ILSAC standard or API Service Category includes the performance properties of each earlier category and can be used to service older engines where earlier category oils were recommended.

Mattax said
Mixing in some 20w50 will bring the viscosity up a reasonable amount for engines that run hotter in the summer.
Switching from 10w30 to a 20w50 is a huge jump in viscosity and would not normally be needed. If the engine is that loose, or bearings that worn, then a 10w-40 in the winter is probably a better match.

Mattax is correct. Excessive viscosity is not beneficial and it better to use an oil that has hot viscosity that doesn't raise the oil pressure to the point where the relief valve bypasses oil flow back to the sump. If you need the viscosity of a 40-grade oil, I would go with a dual rated (ie, API CK-4/SN) oil that also meets Ford WSS-M2C171-F1 like Petro-Canada Duron HP 15W-40.

If you're really concerned about your bearings, read about temporary shear in the Corvair Oil Guide. Look for oils with higher HTHS Viscosity and HDEOs generally have greater HTHS than Starburst oils.