For one thing you will get FLOW pressure that is not intended normally. Another you will get h/p loss from spinning in the deeper oil.
Many others on this forum have lowered trannys and have no problems you should not either.
as for dirty oil... I would change it completely. Perhaps the case came dirty from the rebuilders.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

professional bovine relocation specialist