Have to disagree with nearly everything you said there CL......Engine oil definatley does not attack bronze as valve guides are often made of bronze as are some turbocharger bearings. Some gear oil EP additives though will attack it which is why some gearboxes with syncromeshes made of bronze run on engine oil and not gear oil.
Keeping dirt suspended in the oil rather than letting it settle out in the bottom of the spindle is actualy a very good thing and will minimise wear.
Adatives to engine oil will not form a 'gunky coating' as engine oils are designed to prevent breakdown and seperation even at high temperatures so this will certainly not happen at the low temps found in a TT.
Motor oils are however mostly too thick for many TT applications. The viscosity to be concerned about is the W number which is the viscosity when cold. Film strength is essential to maintain a seperation in high pressure areas like the thrust point and here fully synthetic oils are far superior to old mineral oils and viscosity is not related to film strength. How slippery an oil is is also important with again synthetic oils bettering old mineral oils in this respect however low friction may not be such a good idea as it also acts to damp out drive train variations and a loss of friction could decrease pitch stability. In the end an engineer will have designed a TT spindle bearing with a particular oil in mind. This will take into account friction, spindle to housing clearance, manufacturing tolerance, thrust point radius and material and required damping of the drive system - i.e use what the manufacturer recommends and deviate from this at your peril
BTW ISO 46 is a very thick oil - any links to info to show this is used in the Technics bearing as the close clearance of this bearing would require something much thinner.....