https://www.sheldonbrown.com/rollerbrakes.html
" pack it with high-temperature grease before reassembling"
Sheldon also offers a link to:
https://www.bakfiets-en-meer.nl/2013/01/21/overhaul-time-why-is-my-bike-soooo-sloooow/
if impatient, scroll down to half way and he offers this advice (badly dragging roller brake):
"A fine paste of Shimano’s sacred, expensive rollerbrake grease and road dirt filled the brake unit requiring serious hand force to rotate it.
. . .cleaned it . . .
"No way I’m going to sludge these babies up with that stupid grease again! What else could I put in there to keep them from rusting and lubricate the innards? After a quick inventory of the dozens of little bottles and cans on the Workcycles shelves we decided that a thick, clingy OIL ought to work, even if it violates all instructions, death-warnings and warranties. After all these brakes are fairly well enclosed and don’t see any substantial heat here in cool, mostly flat Amsterdam. The most likely problem I anticipate is that they’ll have to be oiled periodically but whether that means monthly or half yearly remains to be seen."
YMMV, in particular the cool, mostly flat bit. I guess a grease or thick oil which purports to be heat resistant may inform your choice of lubricant.
