Unless it's really crusty, the best way to clean the rheostat windings is just "use". Turning the knob fully both directions several times will usually knock most of the oxidation off.
You can try an electrical contact cleaner spray on the windings.
If the lights smoothly transition from dim to bright (no outages or "hiccups" along the way) - the problem is probably not the rheostat. The age of the wiring and connectors (oxidation = resistance) will play a role in the dimming of gauge lighting.
Many people will install higher wattage bulbs. I don't. Most of my gauges have been disassembled at one point or another and I find that the paint they sporadically sprayed inside the cans is anything but white after all those years.
I pull the guts, respray a nice flat white on the inside of the gauge and even with the stock 3.4w bulbs my gauge lighting is nice and bright. (Well, as bright as they were intended to be back in the '70's... I'm old, I grew up on green gauge light, very little "light pollution" in an analog world.)
1974 "early" 260Z - L28ET - MS3X full sequential fuel and spark, COP=Ford 4.6 coilpacks w/ Nissan Ignitor, diyautotune chopper wheel - FMIC - T04E .63 A/R - 400cc Injectors (too big!), 14point7 Spartan2 Wideband - C/R 5-speed - Coilovers - Big Brake Kit up front, Porterfield friction all around. Mesa, AZ