420ml of oil is OK but you could add a bit more to be on the safe side. I go for 450ml in mine. No point in cutting it too fine while he's learning.
I don't know how much your lad weighs but mine is only 8 and as light as a feather. Consequently, we have moved the fronts out slightly from the recommended setting to help generate a bit more lift on the inside rear. You can also use castor adjusters to help increase the angle and hence the lift. When you do move on to slicks, start at 28psi all round. As he gets faster you can start dropping it but try it very gradually. You might find it runs better with 2 psi less at the back than the front. This certainly works for us now but I don't know whether that will change as my son gets faster.
Finally, my only other advice is to keep the bearings and chain really well lubricated. I thought I was doing this really well and then got one of the other dads to look over the kart and he suggested more. Sure enough, a load more chain lube later and it was clearly spinning much more freely while on the trolley.
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