[Sirrotica]

My neighbor rebuilt a 460 ford in a 1972, mercury, and it did the same thing, the new oil pump was defective and the bypass valve was stuck. He had the same problem as you're having holding the valves open when cold, backfiring etc. As soon as he ran the engine about 30 seconds he got enough heat to thin the oil out, and it was fine the rest of the day. The next morning the ritual started all over again.

One morning I was standing outside at my house and heard the problem when he did the first start in the AM, and after we talked I told him I thought the oil pump relief was stuck closed. The car only had an idiot light, so he had no idea what the oil pressure was. I told him to buy a gauge so he could monitor it, sure enough, over 100 PSI on the first start, and when warmed up it was over 75 lbs at highway speed. Another new oil pump solved the problem.

I hope maybe that incident might shed some light on your customers problem.

A stuck relief can also cause the oil pump driveshaft to shear, I've seen that happen on a 302 ford about 40 years ago, of course the oil light being on didn't make the guy stop driving it, he ran it until the engine seized.