For my car it’s been message boards with enthusiasts and mechanics who work on my vehicle. Luckily (or not…) mine is a somewhat niche performance car so people tend to nerd out over them.

A lot of it though is ignoring the vendor recommendations for outsourced parts like transmissions - go direct to manufacture and see what they say. This does seem to involve having an “insider” with manuals intended for service techs.

I also imagine fleet vehicle schedules might be informative as well.

Unfortunately I don’t have a very clean direct answer on it - it was a lot of feeling around and when in doubt erring on the side of over maintaining vs under.

Some is simply basic common sense though. We have not developed lubricants that can last over a decade and 120,000 miles in temperature ranges from below zero to hundreds of degrees.