I used to do milling and I tried to get to as great a level of accuracy as possible (to the annoyance of the foreman, who was more interested in speed). I remember using a vertical mill with a 6" TCT cutter (that may be the type you refer to - they have removable square TC cutters which clamp into the cutter body). I found this would work very well on aluminium when using a high cutter speed (and with fresh cutters). Faces being out of parallel is often an issue with old Triumph heads. Parallel bars under the workpiece, shimmed with shim steel can get the upper face nicely parallel with the table.
I certainly agree on the oil blackening. On a rebuilt engine the oil should stay looking new after 5 minutes running, and still look pretty much like new after 100 miles or more. Back in those days, diesel engines were regarded as things to be thrashed mercilessly and never much thought about, other than oil & filter changes now & then. They were very crude compared to modern diesels. On a Triumph, cleaning out the oil tank is, I'd think, quite important - can be very important if not done for an unknown period. The gelatinous goop that can accumulate can be shocking!