Won't be an air lock, the de-gas system is excellent on these cooling systems, the way to check for air lock is more by way of symptoms, none of your would point me to that being a problem.
Yours sounds VERY normal, you can't compare a 4wd with the previous 2wd engine, unless of course you are using all the same bits ie 2wd cooling system, stat, sender as fitted before.
Without trying to sound disrespectful, I think you are underestimating the difference even a few degrees c outside temps make.
Just check the turbo water return hose to the expansion tank is nice and clear, as this is the de-gasser.
tabetha
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