That simply isn't the same...
The idea is to have a diff that is nearly locked like the spool but that is much more forgiving mechanically and also easier to drive. Putty or ear plugs or all of those are comparable to some 50000 wt shock oil if they ever needed that! They do sell actual silicone (liquid) diff oil you could use that has 125000 cst in weight. It is easier to fill up the diff with it because it's still liquid but it's still a bit too loose for most racers. Putty is an extra step stiffer than that.
I've tried 50 wt oil in the front and the car drives nice but it's a bit slower that's all.