What does the diff have in it now?
I'm assuming this is for the center diff.
With my outcast (1/8th sized short wb truck), when I ran 4S, I ran 500K in the center and it did fine. Would still wheelie when I wanted to, but would allow enough power to dump to the front tires to make it easily drivable. It came stock with 100K in it I believe and it unloaded to the front constantly, which was really annoying for bashing for me.
When I started running 6S, I dropped it to 300K because 500K was undrivable. It just wheelied all the time. Even 300K is a bit much as it still has a tendency to pick the front end up in inopportune moments when I'm trying to pick up speed to hit a jump. If it was a kraton with longer wheelbase, 500K would be fine probably with 6S, but the shorter truck needs less for me at those RPM's.
If I pull the diff apart this winter, I'll replace the oil with 200K and see how I like that next spring.
EDIT:
Just looked it up, the shredder is a long wheel base truck. So likely thicker than what I use in my outcast will work fine. Just need to figure out what it comes with to make a best guess.