That's because you're interlaced. The base sensitivity (ISO) for S-Log is halved to 800 only when shooting progressive. For maximum dynamic range it's best left on either of these settings.
If you are using paid SLOG then there are no settings for you to tinker with.
If you are using free SLOG (via firmware v1.4 and up) then you must create a custom picture profile and select SLOG under the gamma menu option. This option also allows you to dial in your white balance. You can also then set your L gain setting to -3db to lower your ISO a little but best thing if you are still too sensitive is use additional ND filters.