Just an update for any other unfortunate arctic dwellers, this problem has been 100% resolved by using a winter front. Just a piece of reflectix bubble foil tucked in front of the aux fans, I've taken it out for several long and cold drives with no recurrence of the issue.

Temps with the winter front never exceed 80C unless ambient gets above -5C, and when it does there is plenty of time to pull over and remove it before the aux fans kick in.

The only issue I'm seeing now is excessive consumption of my light winter oil (10W-30 synthetic blend) as it appears to be getting slurped through the breather when the block is not being chilled by the arctic blast. I'm steady draining it out of the catch can. Maybe time to go back to 15W-40 as I run a pan heater anyways.

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1993 300D 2.5L Turbo