Optimum? I would suggest the "ideal" interval is somewhat short of "optimum"; assuming by optimum you mean maximum oil life versus increased damage potential.
There are formula to calculate cost vs risk in oil change intervals; ie cost of oil + labour vs risk of engine replacement. Broadly speaking they tend to promote OEM recommended intervals vs early intervals once you crunch the numbers.
The data you're seeking isn't really available from OEM sources; you need to find fleet data that represents extended intervals.
There have been a few published studies involving municipal bus fleets and military vehicles with bypass oil filtration vs full filter only. You need to find or create from your own data similar studies.
Cost of engine replacement (all costs, including downtime and costs to your reputation or loss of goods carried) prior to your intended retirement mileage (could be open-ended) = X
Cost of maintaining oil change regimen (oil, labour, downtime, whatever you find you need to put there; monitoring if you go extended, etc) = Y
When X is less than Y, your oil changes are too expensive.
When X is greater than Y, your oil changes are sound practice.
The ratio of X:Y over different regimens gives you your optimum.
But your ideal may be another point. The reason is X is a rare and expensive event with less statistical consistency while Y is an incremental cost with high statistical consistency.
So, you should guard against X but monitor Y. Reducing your interval from optimum would be the way to do that, and if that resulted in, say, a 10% increase in oil change costs that is small in comparison to the cost of motor replacement (assuming X is greater than Y).
If X is not greater than Y, you are entering the realm of a disposable motor.
