krellmk,
To try & give you a definitive answer to your specific situation I'd first need to know the initial conditions that you started from.
However, to answer your question in a general sense I can use the following example to show how a good synthetic oil (of the right viscosity) could make an audible improvement in the sound.
Let's say that we have a vintage turntable where, over a period of years, the original bearing lube has the lighter parts evaporate, leaving a higher viscosity 'sludge' behind. Now it takes more torque from the motor in order to turn the platter. When the specific motor in this turntable has to work harder, it starts to 'cog' at a 60hz rate. (An electric motor
'cogging' from magnetic field to magnetic field is similar to 'lugging' a gas engine -- you end up superimposing a vibration on top of what is supposed to be a (relatively) smooth torque delivery. (If mechanical rate of rotation was a DC voltage, this 'cogging' is just like having AC ripple on top of that DC voltage.) Think of this cogging as a 60hz bass 'warble', and what these fluctuations would do to the quality of the sound.
Of course the other variable is the weight of the platter (flywheel effect) vs how much the motor vibrates from cogging when working harder than it is supposed to. (For example, an old Dual turntable I used to listen to had a 7-lb platter, and the motor was stout enough to get it up to full speed in <1/2 revolution.) When I bought a much newer direct-drive version, most of the platter weight had been removed...and it just wasn't quite the same. (And no, I didn't have the necessary lab equipment to quantify the difference at the time.)
****
So, getting back to your question, in this area it's quite possible, but how much of a difference depends on the design of your motor + the weight of your platter. As for which oil is the best, I would first figure out who made the bearing, and then see if there is any application note out there telling the purchaser what the designer recommended. Assuming that info is lost forever, the next best thing would be to perform some empirical testing -- that is, try 2 or 3 different oils (or the same oil at different viscosities) ...and whichever one gives you the longest spin-down time (from 33 1/3rpm to 0 using a stop watch) ...then that is the one that I would assume will put the least drag on your motor. Less drag = less cogging.
****
The above assumes that the drive motor is the
largest source of inducing a amusical 'masking bass' on top of the bass content in your music.
There are other turntable designs out there where the motor is actually quieter than the rumble from the platter bearing itself. In this case, then experimenting with different lubes will help you find the one where the bearing itself runs quietest...and in this case, the thinnest viscosity may *not* be the quietest solution.
Q: Does this explanation make sense?
3D