c0sie,
For a slippy track and 4wd then it really would depend on the conditions (and what motors you have). If it's just a bit slippy but a big track then the 5.5L would just take the edge off it. If you need something softer all round then dropping to the 6.5R should be good. A 6.5L should be VERY driveable in any 4wd worth its salt.
Regarding indoors I find it difficult to imagine a track from the numbers. What I would say is that it's unlikely you''ll need anything over a 6.5 in 2wd. I recently ran the 5.5L at Teeside (it was in the car at the start of the day) and new it was far too much (hard work on slippy floor) but was busy building an X5 and couldn't be bothered to change it. I know Little Ben ran a 7.5 and clearly wasn't slow as he finished quite high in the A. So perhaps less is more, especially on tyre wear
All the above comments don't count for somewhere like Worksop, as that's the size of an outdoor track but indoors and pretty grippy. I've run both the 6.5L and 5.5L there, I know which is more fun but not sure which is quickest lap time wise
Roger I'm in Wales, remind me to reply after next weekend when back at home with cars.
When they are built/rebuilt for next year my 2wd will have the Spread Spektrum GTB and 6.5L/5.5L motors in it and the 4wd the Spread Spektrum GTB and 5.5R motor in, I think that will be great performing and reliable.
Dan