Synthetic oil use on flat tappet cam after break in

I won't wade into the discussion on which is best in terms of lubrication, but I do believe synthetics are more likely to leak. That's based on my own experience(my own vehicles) as well as the many we used to see come through the transmission shop.

Does anyone remember how the first synthetic oils looked black, like liquid graphite? Back in the 70's, my father was working at a large motor manufacturer, as he did for 48 years. One of the big oil companies bought a big motor of about 10,000HP. Takes about a year to make. During production, they came in and said they wanted the motor bearings to be filled with the new synthetic oil they had developed. The motor engineers naturally shriveled up at this idea and told the customer 'We'll do it but we won't take any responsibility for the bearings....' The oil company said 'No problem, this oil is hot **** and we know it'll work'

So, when it came time to test the motor on the shop floor, they filled it up with the new synthetic oil and spun it up. After the machine ran a while, they started picking up all the usual sign of an unhappy situation so they shut it down and lifted the bearing caps. The oil had cooked itself into a black carbon-y sludge. After the head scratching was over, they swapped out to all new bearings and refilled it with conventional oil which ran fine. That's a page from the early days of synthetic oil development!