I wouldn’t really know what to suggest specifically for grayscale.
But in the lossless department, you can check out some of these
http://compression.ru/video/codec_comparison/lossless_codecs_2007_en.html
(download the pdf and read - it’s pretty easy to understand)
I think TechSmith’s another lossless one not tested on there.
Lagarith seems to be the winner on speed, followed by Huffyuv (probably the most widely used of them all) and Fastcodec.
You may want to ask the experts at http://forum.doom9.org their opinions
P.S: Come to think of it, grayscale could change the whole thing since there’s only one channel to worry about (instead of 3 in RGB or Y’CbCr). 
I need a lossless codec that supports grayscale, with good speed/compression (more bent on speed).