Hi Andrew,
I find that running the 0.5mm, lower & wider, insert in the FF and FR blocks generally gives more overall front end grip. It seems to increase the amount of steering you have in the car, without making it too twitchy and hard to drive.
As you say, by shimming the top deck, you are introducing more flex to the car. This too helps improve the car on low grip tracks.
As far as diff oil is concerned, I don't particularly think that thicker oil is better on low grip tracks. In this instance I used 4k oil to give more on power steering, which was needed for the longer corners on the CWIC track layout.
I tend to start around 3k for most tracks, and then adjust from there dependant on feel and grip levels.
I Hope this answers your questions, let me know if there is anything else I can help you with
Zak