The Disco warms up very quickly... And runs pretty hot.

But when you start it at -20 the oil is very thick. I’ve been using 5-40 in my climate (British Columbia southern interior)

Well, yes, I don't know if you mean -20 C or F, but in any case, that's a whole different environment. Here in D.C. area it still very warm, and will be warm for another couple of months, give or take. Winters here are mild, temp are mostly in the 30s F during winters, as well as 40s. Not nearly the same climate. I've lived in places where it's often -15 -25 C during the winter, and in that case, you want to use a "thin" oil, 0w50 or 5w40 or something like that, for sure. I probably could have used 5w40 here also, but Mobil1 10w40 Full Synth Hi Mi was only $25 for a jug of 5 quarts, and it's one of the oils that most people (and the manual) agree is on of the proper oils for our engines.

The weird temp gauge in the instrument cluster suggests that, but not when you measure oil temperature. I have such a gauge and It takes me 10 minutes in city driving until the the oil is at operating temp.

It's not wrong to use a 5W instead of a 15W oil in warmer climate, oil is just more expensive.

What kind of oil temp gauge do you have? How difficult to install? I'd like, in an ideal world, to have an oil temp gauge also ))

You can have an oil temp gauge from the OBD2 port with an ultragauge, scangauge or OBD app for your phone like OBD Fusion or Torque Pro.

The 10w40 oil is good for -10C which is 14F and up to 50C/122F according to the owners manual. Yes, in colder climates a 5w40 would make sense.

1/8" NPT temp sender at sandwich adapter on oil filter. It connects to a DIY unit (Arduino & Bluetooth). To display I use the Torque Pro app. This sandwich adapter has two ports, I use the other for oil pressure.

https://www.bmotorsports.com/shop/pr...oducts_id/4176
https://www.glowshiftdirect.com/oil-...wich-adapters/
https://wiki.torque-bhp.com/view/Arduino