Oil for Gen 4 Jimny

Just out of curiosity where has this idea come from because it really sounds like the sort of thing you get told by a mate of a mate who knows a man in the pub who once cleaned the canteen at the Dagenham Ford factory and knows everything they don't want to become common knowledge? I ask because I was told by euro car parts that my land cruiser needed 5w40 but it ran like garbage after that oil change and upon investigation it was supposed to be 0w20 from memory and after another change was back to normal, they also got me with an alleged gl4/5 compatible gearbox oil when I had a manual and once warm that wouldn't change out of 5th, swapped for the correct gl4 and back to normal.

Sometimes manufacturers get it wrong.  Owners of original MX-5s often suffer with stuck hydraulic tappets (HLAs) and Mobil 1 0W40 seems to be the best way to keep the problem at bay, once you've cleared the offending tappet(s) with Wynn's hydraulic lifter treatment or, oddly, a shot of ATF just before an oil change.

Using the 10W30 mineral oil (I think), as Mazda recommend, means the problem comes back straight away.  It doesn't have the penetrating and flushing ability.

But there are no reports to suggest that the jimny is coming out of the factory with any kind of oil or lubrication issues and some of the earlier gen4s are really starting to gain milage now.

150 miles later, no issues yet, at least nothing i can notice

Using 5w/40 oil in a Land Cruiser engine shouldn't in itself make it "run like garbage"
Unless you are at the North Pole.

There may possibly have been some other issue with it and it was just coincidence the oil was changed at that time.

Dunno but it made it quieter and easier to start and the symptoms came and went with the wrong oil. But unless the dealer can fix the electrical leak it's on borrowed time anyway.