I'd think 18 ga wire would be OK. The pins and the lead to the temp gauge aren't heavier than that, if I recall.

Behavior afterwards? The big difference is that it won't drive you insane.

Mine stays pretty close to center once it comes up to the midpoint. Mine rarely goes more than 1-2 mm. above the line at warmup; others report it goes high and then back to center when the thermosat finally opens.

When driving it may go ~1 mm. or even 2 mm. below center if on a downhill with cabin heater running. That is, low engine load and lots of cooling for the engine.

In summer on an uphill it may go 2 or even 3-4 mm. above center, and come back down on the downhill side of the hill. Of course that depends on how fast you go up the hill. If you don't step on the gas too much and let it climb the hill slowly, the temp won't climb too much.
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Sven: '89 245 NA, 951 ECU, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors, quad horns. Wifemobile '89 245 NA stock. 90 244 NA spare, runs.