Quote Originally Posted by gavinwibrow View Post
well under the 60-65C I occasionally get to under heavy towing, and which according to Ashcroft's is actually lower than preferred
Wow. Ok, so I fitted an external thermostat to the Volvo because the old AW box doesn't have one and with an Aux cooler in winter driving around town unloaded the oil never warmed up. I was under the impression that most boxes made in the last 20 years had cooler thermostats built in.

If those are your bulk trans fluid temperatures then yes, I'd fit the thermostat. While I said the radiator *never* imparts heat into the fluid under running conditions it's never going to pull the fluid temperature as low as your cooler can as once the thermostat is open even the cold side of the radiator will be considerably above ambient. It sounds like your cooler is quite impressive and is quite capable of over-cooling the fluid, so a thermostat sounds like a good idea.

There's 2 reasons. First, the obvious one is cold oil is thicker and some of the anti-wear additives work better with a bit of heat.
The other isn't as obvious until you think about it. While the transmission is effectively a "sealed" hydraulic circuit (unlike a crank case for example), the fluid still expands and contracts (a lot). This draws moisture in, and getting the fluid up to temperature speeds the process of the fluid rejecting the moisture.