It seems that f/4 lens can get a bit too dark using Canon Eg-S screen. On this forum, one user said:

I tried 24-105 f/4L and for indoor uses or outdoors at evening I believe VF is too dark. It's still usable but the VF is really much darker than with standard focusing screen

Perhaps, better question would be: I have f/5.6 manual lens and would like to get better focusing screen for my Canon 5D II

Split-prism focusing screen might be a good option. Unfortunately, Canon doesn't make one for 5D mkII. From their PDF on screens:

However, there is a Taiwan company that seems to be selling split-image screens for Canon 5D mkII (and many other cameras), for ~$85 USD:

It seems that f/4 lens can get a bit too dark using Canon Eg-S screen. On this forum, one user said:

I tried 24-105 f/4L and for indoor uses or outdoors at evening I believe VF is too dark. It's still usable but the VF is really much darker than with standard focusing screen

Perhaps, better question would be: I have f/5.6 manual lens and would like to get better focusing screen for my Canon 5D II

Split-prism focusing screen might be a good option. Unfortunately, Canon doesn't make one for 5D mkII. From their PDF on screens:

However, there is a Taiwan company that seems to be selling split-image screens for Canon 5D mkII (and many other cameras), for ~$85 USD:

PS: on the other hand, I wonder how much precision you need to focus f/5.6 lens, even a longer one. Depth of field will be larger, so I guess there is less pressure to focus precisely, unlike with paper-thin DoF of f/2.8 tele and f/1.2-f/1.8 normal lenses.