Short of pulling the engine and EVERY oil-bearing hose, fitting and appurtenance apart and thoroughly washing each bit, you'll never be rid of the grit.
Likely the tank was sand/bead blasted before painting, without properly washing it afterwards.
If no intention of tearing the engine down:
You can always thoroughly flush the tank with Simple Green, scrubbing with a bristle brush, then high pressure car wash flush. Then, pull off ALL oil lines and fittings, replace the lines and wash the fittings. Then, flush the engine with kerosene from top to bottom (remove the timing and rocker covers to help), sticking a thin bottle brush in wherever possible. Then flush it again, and again.
Install a new oil filter, start up the engine, run it around the block and change the filter (and dump the sump). Check for grit and repeat if you still see any.
The damage is already done.
author "Old Bikes"
too many bikes to list, including:
JPN, 952 Prototype, MkIII Interstate, Combat Dunstall, VR880 SS, Triton