You have no clue. I have wrenched for 30 years on everything from MG Migets to 150 ton rock trucks and done lots of field service. Now 10# idle oil pressure is not optimal. A taxi that idles half the day waiting for fares and then putzing around town just fast enough for OD to engage will not produce long life. One advantage taxis have is they do not drive short distances, get shut off and allowed to cool down again in -35°F weather, like personal cars. Cold starts are tough on engines.
Next question, how many of those engines that you say had low oil pressure caused rocker shaft wear, also had a lot of miles on them. 90% chance most did. Many start with worn main and rod bearings which allows oil to leak out. Now oil to the cam bearings is reduced, allowing them to wear. At the same time the rockers are wearing on the shafts.
I have rebuilt a lot of engines and repaired a lot also, and have also seen all kinds of failures. I have also rebuilt automatic and powershift transmissions from TH200s to the 600HP capable Allison powershift that goes behind 750HP 12V92 mining equipment and also some big ZF gear boxes. I do have a clue.
Detroit Diesel had a problem with some of their Fuel Squeezer 8.2L V8 engines breaking the socket on top of the push rod. There would be a polished ring close to the top of the cup and part way up the adjuster ball. Some came off a different machining line with a slight difference in diameter. The bigger ball would eventually split the cup.
Freightshaker had a problem with the air system on new trucks yesrs ago when spinon air drier filter canisters came out. The compressor governors were not unloading the air check valves and pressure would go up to 250 PSI. The plastic line to the wet tank would get hot, swell like a ballon and rupture. On one in the middle of the night, the drier cartridge blew right through the fibreglass hood. Scared crap out of the driver on a long steep hill in the Roger's Pass on Hiway 1 in the Rocky Mountains near Golden, British Columbia. Try getting a heavy load rolling again on a steep hill. The problem was Freightshaker had plumbed the pressure sensing line from the governor into the wet tank, instead of the dry tank. This allowed moisture into the sensing line which froze up in the cold. Had a dilly arguement with the Freightshaker engineer sent up to see what the problem was. Funny thing was, when I plumbed it into the dry tank no more problems. On pre-delivery inspection after that we ran it to the dry tank and no more problems.
