Id take it to a 4x4 shop that specalize in Patrols, I had a mechanic tell me once because my intercooler was oily I had "blow by"
total crock of droppings, As for your mechanic try another, exhaust and water temp are not related. When you have your
gauges fitted like Mudski suggests you can "drive to your gauges" meaning if you see the EGTs getting hot back it off a bit.
I NADS my motor at 185K thanks to a member here [Boots], I have not had an issue with it since, I occasionally tow a small camper
up to the Murray river with no problems at all. Get your gauges fitted ,that will tell you whats going on and then you can adjust any abnormalties
with your NADS