T28,
your oil temp stays at 70C?
I reckon your oil temp sender is near the oil tank, IN the turbo while it is glowing red the oil will be much hotter. That is why a slow cooldown [not a really long idle period] is needed to allow some time for temps [turbo, cylinder, crankcase, oil] to stabilise.
Yep, 30 mins after stopping the oil will still be quite warm.
Engines like being heated and cooled - slowly. [but I am sure I am preaching to the converted]