The greased spindle bearing is probably worn out. however, you can run it with a higher viscosity oil to compensate until you decide what to do with it.
I am running low viscosity atf in my south bend 9 spindle bearing (1.815 inch diameter, 2.25 inches long, vertical clearance is 1.5 thousandths), its viscosity is 6cst at 100C vs the 4cst that velocite 10 is at the same temperature. However many others have reported using 10w30 engine oil in the same spindle and its viscosity is 12cst at 100C, but they probably don't make a habit of running it at 1500 rpm either, and at that rpm my spindle warms up by perhaps 20F.
If you can't get the spindle bearings fixed for whatever reason, then i would recommend using the thickest oil that does not heat up at the rpms you intend to use the lathe at, so 20w-50 is appropriate if you can't get rid of say, .005-.01" of slop in the bearing. --though its going to heat up regardless according to the load, and friction, eccentricity and whatever else is going on.
more clearance and thicker oil makes for more friction than thinner oil and lower clearance. but you can't use thin oil in a high clearance bearing... this is why automobile engines are starting to use very close fits on the bearings and 0w-20 oil. but once those bearings wear out you would need to use thicker oil to prevent a destructive spiral from occurring.
