I've been running unleaded gas in my 360 with non-hardened J heads since I built the engine in 1993. Daily driving duty for long periods, long highway trips, you name it. I've probably put 150,000 miles on it.

I recently tore down the heads. Major valve seat recession, and pitted exhaust valves as expected. BUT, it was still running fine expect for the fact that I wore down a cam lobe as a result of coolant in the oil, rusting a lifter in place and preventing it spinning on the lobe as it should. No cloud of smoke etc, no dramatic loss of power.

The seat recession I attribute to a larger than stock cam and valve spring. With every valve closure it is like a tiny hammer strike on heated metal. The softer metal is going to move.

The valves themselves had the bigger part of the damage. Stock exhaust valves from rock auto cost me about 6 bucks a piece, a little lapping compound and elbow grease on the valve lapping tool and they should be set for another 100,000 miles. Compare that to the cost of installing hardened seats, which if incorrectly installed can drop out and hole a piston. I installed adjustable rockers (which I planned to do anyway) and they were able to correct for the valve seat recession. Otherwise a trip to the machine shop to trim off the tops of the valve stems and set the tips at equal heights would have been the route I took.

Is my rocker geometry perfect? No. Are all of my valve spring installed heights within .001? No. Is it what you want to do for pro stock or FAST racing? No. But for a street driven car, with a mild cam, my car is running just fine still, thank you very much.