I don't think there's a niche market for muscle car era oil at all. Except for on display racks like Dog's (and mine). Owners of these rare cars want the very best oil they can find, and today it's the HD diesel oil like Rotella with the best additive package to protect those old bearings and journals during storage, cold starts, short run times, rich, sooty combustion by-products, moisture and the high vapor pressures of today’s fuels washing down cold cylinders........JMO.
Verne