There's not a certain mileage to my knowledge but when I had a hr I had them replace at 69k.
The way I diagnosed it was off the oil pressure gauge. Mine was idoling at just over 0 when warm.
horsham developments have tool which they can diagnose if the gaskets have gone.
Welcome Mas, I've owned several zeds over the years. In all honesty I'd go for the 370 if your budget allows.
Just bare in mind the oil gallery gaskets will need replacing on either the 350hr and 370z if they haven't been done already.
I was wondering if anyone would be interested in visiting H.R.Owen specialists cars in Cheltenham as part of our next meet?
I've spoken to them and they are willing to let us go down on a Saturday at some point and have a look at their stock.