The wt rating does seem to vary....I have used racers edge and Associated since that is what is stocked locally.
I am starting to go all associated on my shock and diff oils. I like their bottles better and seem to be more consistant, I had a bottle of 40wt Racers edge and was almost out so I went to get another and noticed a very obvious difference in consistancy. Same brand purchased maybe a month apart and they appeared to be very different. It may just be my imagination, but I raced on an unusually cold night and it seemed to affect my car with the Racers Edge oil in it more.
So I have been sticking with the AE oil, its stocked locally and seems to work fine. I figure working on my driving skills will make a bigger difference than the brand of oil I use anyway