There have been a couple of previous threads about this.
I seem to remember Pukkasilvia saying that you don`t really need the water cooling, as when the water is hot, it`s not really doing much to cool the turbo anyway
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I guess as long as your oil is changed regularly and there are no blockages in the oil supply pipe you should be okay. I know that a few production cars don`t have water cooling for the turbo.
Hope this helps
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Regards,
Ian.