I've got the 2020 TRD Off-road and I've read the features outlined over on trail4runner and other places, but I almost get a bit scared where it seems:

1) You should try to "regularly" drive in 4HI for a bit to keep everything working well
2) You should absolutely not use 4HI on dry pavement and risk damage.

So.. what's sort of the baseline for safely using 4HI? Can you use it on a road when it's actively raining, even if the pavement isn't pooling any water?

What level of "non-pavement" can you use it on? For example, there is this pretty well maintained service road:



It's not pavement, but is generally in pretty good condition. But there could be gravely spots, definitely some potholes, yeah...

Hey there and Welcome! You really don't need 4Hi in the rain on pavement, snow and ice you might need it. You can still create binding in the 4x4 system when turning sharply even in the rain, and the traction control in these vehicles will help quite a bit on wet roads in 2wd anyway.
The gravely road you pictured might be ok for going slowly on in 4Hi as long as your not taking sharp turns. Now that same road in a heavy rain is where you would want to use it.
Using your 4x4 system definitely needs to be done several times a year to keep everything working properly for years to come.
What I've been doing recently since I haven't been able to get out to the trails lately, is heading over to a road by my house that is not heavily traveled, and has a good dirt and grass shoulder off to the side. I will engage the 4x4 and just slowly drive the shoulder for a mile or so and turn around without getting on the pavement just drive back down the shoulder. I might do this 5 or 6 times, basically looking like an idiot, but I'm exercising my systems. Because you have an Offroad model, this will include both your 4Hi and 4Lo systems as well as your locking differential in the rear axle. If you can alternate the hi and lo system a few times and lock in the differential as well, this is good for the servo motors and the systems that engage the gears. Think of the servo motors like an electric window motor that never gets used. Eventually it will not work correctly from corrosion and sitting around too much. Doing this every couple a months will keep them working properly. But, I highly encourage you to get out there off road on some trails and use your truck like it was designed to do. It's a blast! Hope this helps and Good Luck!!

So as far as the routine of using 4hi for just getting things moving and lubricated it is ok to use on wet pavement and best just to use when you can exercise it on relatively straight stretch of road the less turns the better and for sure no sharp turns. Service roads such as loose gravel and dirt it is probably ok for 4hi, if it’s needed. Basically, if it’s dry and there is too much traction you won’t need 4hi anyway but if you have it in 4hi and there is too much traction you will get binding and you will feel it when the 4wd is unhappy for sure. No need to worry too much it’s a fairly robust drivetrain that can take quite a bit of abuse so if you hear feel it binding simply put it back in 2wd.

If it's been a while since I hit the trail, I would not hesitate to put it in 4-HI, on a relatively straight stretch of dry pavement.......I do it all the time, no problems.

As stated above.... a long as you are not making sharp, parking lot, like turns in 4wd, on pavement....you'll be fine.

Okay thanks guys, it's easy to get overwhelmed when some of this stuff is new to you and there are official and anecdotal warnings everywhere!

I was just thinking this same thing as the OP. I too just got a 2020 ORP. Not sure when I will be able to hit some trails. Straight stretch of road on 4hi is no problem. Can the same be done for 4lo at slow speeds on pavement? What's an ideal frequency to keep things optimal?

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Many of us have forgotten to shift from 4wd to 2wd when back on pavement.

It's undesirable, but not the end of the world. The steering will feel heavy and a little vague, and when making sharp turns, you will hear the tires scrub.

So don't do it, but don't worry about it if and when you do it.

Straight, dry roads are fine. Just remember to disengage it when you need to turn because it will feel like your Runner is broken as the driveline binds up. I do this a couple of times a month without issues. I also select low range on empty back streets and exercise the locker. Just get it to where it locks in and you can just unlock and lock it on the spot. I don't always have time to get to a dirt road for this.

I also used to 1/8th mile drag race my supercharged Tacoma in 4wd. No issues ever.