Just saw your episode entitled "If You See This, Drain Your Engine Oil And Pray" In the section about leaving something plugged into the OBD-II port. I have an Ultragauge. I got this because I like a little more instrumentation instead of idiot lights to tell me how the car is running. Idiot lights only tell you when something has gone wrong. I've had it plugged into the OBD port for almost seven years with no noticeable detriments to the running of the car. I love that I can monitor the coolant temp, MAF flow, torque, whatever I want that the ECM monitors. There is really a lot of useful measurements that I can monitor in real time, as well as having it tell me if a CEL code has been tripped. So. Do you have any opinions on the Ultragauge (www.ultra-gauge.com/ultragauge/index.htm) and if it would give you the trepidations you mentioned in your episode.
Since you requested it. The vehicle I use it in is a 2015 Mazda 6 with a 2.5 Skyactive engine and a six speed manual transmission.
(Yes I am that old school).
I don't see any problem with that.
I keep my Scanguage II plugged in. I think the issue is with poorly designed products, Chinese knockoffs, and those scam gizmos that claim to save fuel. Just throw those right in the garbage. Realise that the car's control modules are plugged into the same circuits all of the time.