Viscous fan oil causing cracked heads?

[oneidadan]

OK now I might be talking rubbish (again) but I've picked up from a landcruiser forum about how japanese imports only have a certain type of oil in the viscous fan that's designed for Japanese speed limits of 50mph whereas a different type of oil is needed for our motorway speeds, thus leading, in the long term, to overheating and cracked heads.
I believe it mentions the issue on the routrax website under the LN130 viscous coupling (my right mouse button is broke or I'd link to it here).
I've not researched it and I've only just read about it so it could be a well known thing that I've just never read about but if not could there be any truth in it?

[BUSHWHACKER]

It's poor head design thats causing the heads to crack, the casting between the combustion chambers and the coolant passages is too thin.

Above 40mph, the engine fan isn't doing much anyway, it's superseded by airflow through the radiator.

[oneidadan]

Excellent thanks. Just out of interest though are there different types of fluid for different speeds?

[BUSHWHACKER]

Excellent thanks. Just out of interest though are there different types of fluid for different speeds?

You can get a higher viscosity oil to 'stiffen' the fan clutch to keep it spinning via the engine for longer after the engine gets up to temp.

[NiftyNev]

It's poor head design thats causing the heads to crack, the casting between the combustion chambers and the coolant passages is too thin.

Blame it on global warning.

Nev

[TonyN]

I thinks more important to check your viscous fan is actually working, lack of fluid, broken/jammed spring, etc... will all cause it to not drive properly.

I use a more viscous oil when rebuilding them like Vince said, you can get it from R/C model shops.

The fan is pretty complicated to make it work just right, you want its driving the least amount possible for fuel economy, but enough to keep rad cool, so by monitoring the temp of the rad with a bi-metallic spring on the fornt of the hub, it alters the flow of oil in the hub to speed up or slow down the fan.

Any slippage of the fan will result in higher engine temps, there isn't a lot of spare extra cooling in stock the Surf set up, its easy to overheat them if anything isn't exactly right.