I have not opened my bearing yet to change the oil, following the philosophy of “if it ain’t broke, don’t fix it”. It’s a different situation here than a car’s oil. The seals prevent the oil from being exposed to oxygen, moisture and other contaminants, maybe not perfectly for 30 years though. There is no degradation from heating cycles such as in a car. Some of the more volatile components in the oil could have evaporated, even through good seals after 30 years, though. My seals show no leakage, but if they did, the situation would be more dire. But Jam may convince me to change my mind.
A caution: Micro Seiki used lead-powder in their oil. The dark oil that you see in the bowl in Jam’s picture is therefore quite toxic. I’m betting that the dark color is not due to bearing wear. Use vinyl gloves and a disposable bowl. Then there is the question about whether or not to add lead powder to the new oil. Micro Seiki didn’t add the lead just to make it toxic; they had a reason. Some say it adjusted the viscosity, while others say it filled the tightly-machined voids. I do not know, but its use was certainly intentional.
Here is a snippet from an Audiogon post:
“Micro-Seiki used lead (Pb) "powder" in their oil -- it is very toxic, and should be handled with care. Before you go through the exercise of removing the oil and changing it, you should listen to bearing housing base with a stethoscope -- if the bearing moves freely,and is not noisy, just use it as is. Otherwise you will need to replicate the same original treatment - and use powdered lead (Pb) [available here - www.testbourne.com/materials/metals-details/3523-640/Lead-powder/ ] in your replacement oil. This is due to the spacing in the bearing, and I am not sure if graphite powder would be a good substitute, as graphite might etch or corrode the stainless steel -- whatever you do, use a "high molecular weight" oil -- or an impregnated one.”
A Google search will turn up other references to the lead situation, no doubt.
Finally, a lot of people seem to think that the test of an ideal bearing of this type is that it exhibits zero friction. That is not necessarily desirable. It is possible that some SMOOTH friction (not “scratchy”) that is created mostly by oil viscosity would damp angular (rotational) resonances. It would also load the motor slightly, which could be beneficial (or not). Also, as I recall, this bearing had some sort of pumping mechanism that circulates oil to keep the upper parts lubricated. I’m not sure about that though. That pumping action, if it is working properly, adds more viscosity friction due to the work required to pump the oil. It is theoretically possible for an underfilled bearing to show lower friction (at least in the short term) since oil cannot be pumped. So there is a lot of uncertainty and some conjecture here. My real point is that changing the oil brings up new concerns.
Jam, do you have insights into any of these points? Did you see any grooves inside the housing to indicate an Archimedes-screw-type oil pump or any other pumping means besides capillary action?