Gary I am not against modern oils and was offered an oil by a local parts shop that was the correct grade but was a modern type of oil that was not the one recommended by Penrite Australia that I have used for other cars of this age in New Zealand. I contacted Penrite about this and the reply was that the oil for modern engines has an additive that keeps the dirt in suspension so it can be removed by a full flow filter. The oil they recommend for engines without full flow filters has an additive that does not hold the dirt in suspension so it drops out into the sump. The only different between the oil for modern engines and the one recommended is these additives.
There can be a problem in using oils designed for modern gear boxes and axels in older vehicles, one of them is that EP oils have an additive that attacks one of the metals in bronze at high temperatures and bearings and gears made of bronze wear very quickly or crumble into small pieces. Years ago I worked for an engineering firm the sold worm drive boxes for industrial drive systems 3 of the oil companies I contacted to see what oil our customers could use said to use EP oils as the temperature in the box was not high enough to affect the bronze the forth one put me in touch with their chemist who agreed me about EP oil for this use, the problem with the first 3 they were only checking the temp of the oil in the box not at the gear contact point which is much higher.
Modern oils are very good for what they are designed for but they can cause problems when used as a substitute for other oils
Tony