Best film for long exposures

[Herbert George]

After experimenting with long exposures in earlier days with my Nikon F65 I soon learnt how expensive CR2 batteries can be especially if you chew through a pair after only a handful of exposures. To combat this I devised a plan to first compose star trails/long exposures, attach lens cap, open shutter, remove batteries, remove lens cap ever so carefully, busy myself for however long for exposure, reattach lens cap, install batteries, listen to auto film advance wind on to the next frame.

I probably did this to three frames out of the total 36 odd exposures available to the Fuji Superia ISO 400 roll (cheap and widely available...though evidently not very good for long exposures), only to find upon having my films processed I had three or so "empty" frames. I eventually discovered that my well thought out plan had included first testing the battery removal theory with film removed to positively establish that the mirror indeed remained open even with batteries removed. However the "blinds" behind the the mirror did not remain open and that my hours (literally) of patience had been almost a complete waste of time to the exception of gaining a better understanding of the inner workings of my camera.

In answer to this remaining issue of chewing through batteries at a great rate, I researched, located and purchased a Nikon 'FM' fully mechanical camera and additionally bought a release cable with locking mechanism as well as a compatible wide angle lens (with integral aperture ring). Having done all of this I have successfully taken 'exposures' but still used Fuji Superia ISO400 film which produced very grainy images (as would be expected with the moderate ISO and the long exposures).

My question is...does anybody know of a colour film that is relatively low in grain and that perhaps performs well under extra long exposures?
Can such a film exist and is it going to be so exotic I have to ship it from a remote area of the Soviet tundra?

Attached is a flatbed scan of a print from the aforementioned film. I know it's not ideal having a flatbed scan of a print...I'm working on getting a decent negative scanner.