On my next transmission servicing, I plan to switch both the cars to Valvoline High Mileage ATF.
It's a perfectly fine brand and it meets the relevant specifications for that era (Dex/Merc).
The people at the shop I normally have do these weren't sure about that. They said they like to order AC Delco trans fluid, but I told them "I'm starting to see maybe a little hesitation in the torque converter and I think bumping up the viscosity just a hair and using ATF with more detergents might help with that."
If the transmission gets really bad some people drain out a quart of ATF and replace it with Lucas "Transmission Fix", but we're not there yet with either car. Not by a long shot. Mom has never gotten one of these old GM transmissions past 95,000 miles, and I said "How often are you servicing them?" She goes, "Servicing them?" No, she waited until they blew up and got new fluid then, which came with the trans rebuild.
The Valvoline MaxLife ATF viscosity is not that much "thicker" than regular ATF. If you look at Lucas, it's basically ATF that's thick as molasses that mixes with the other 12-15 quarts that these older cars usually have, and it results in something THICC THICC THICC. 
The idea is it compensates for horrible transmission wear and it might start shifting again.
I've used Lucas High Mileage trans fluid in the 1996 Crown Victoria, but not in the transmission. I drained and refilled the power steering pump with the Lucas fluid, because those cars specified transmission fluid for the pump. The pump was squeeling like a pig when the weather got cold and after the Lucas it stopped doing that.
Don't make me regret that my car takes 7.3qts of synthetic 5W30.
5w30 is affected but it seems that the shortage is mostly hitting worse at the 0w8,0w16, 0w20, and 5w20.
Some dealerships are now quietly using 0w16 in place of 0w8 on some cars that come in, *if* the manufacturer says it isn't ideal but you can get away with it in a pinch.
I was talking to mom, she took my advice and got a couple 5 quart jugs and filters in 0w20 and 5w30 for the Honda and the Chevy
She started using Valvoline MaxLife on the Impala at my recommendation because a mechanic told her it had oil leaks that they wanted $3500 to repair.
She took my advice and at her next oil change, it was still at the top hash mark on the dipstick. It completely stopped the leaks.
I've had so many cars develop slight oil leaks on multiple gaskets, but with GM the most common ones are the oil pans (seen it happen on five GM cars I've had) followed by the front and/or rear valve cover (the 2003 had to have both replaced, but one was free due to a safety recall), and the 2008 Buick had to have the rear replaced), then the 2008 Buick also had oil come out of the timing cover which is the only GM car I've seen do that.
Since the timing cover was so horrific and the oil pan gasket had to come off and so did the camshaft seal, that all became one job on the Buick as well as a new timing chain, water pump and harmonic balancer.
Mom's 2011 Impala was just weeping from the oil pan and front and rear valve covers. It's so old and so high mileage that I understand that she did not want to get into a $3500 repair bill.
Valvoline products are the only ones I've seen do anything at all differently when you switch to the high mileage version, and I say that as the owner of multiple cars that have turned into oil drinkers and this usually works without a massive repair bill.
So if you have any cars like this, I'd say hurry up and stock up on MaxLife Synthetic before it's gone. They also have a MaxLife specifically for over 150,000 miles now apparently.
That would be my major concern of them running out of a particular type of oil, personally. It's not that I think you won't find some sort of 5w30 eventually, but if your car has minor oil leaks, they might specifically run out of MaxLife.
7.3 quarts? What do you drive!?
That's well past the usual 5-6 qt cut off on the oil change base prices if a shop lists a price. They usually say "Oh if you need more than 5-6 qts we're charging you another $10 a quart." even if the oil change special up to the 5-6 quarts was only $26. LOL
I remember having something like this that called for 5.5 quarts of oil and a shop cut you off at 5. So I told them do the oil change but put a 5 quart jug in it and then I went out there and put another half quart in, in the parking lot.
Driving a 5.5 qt system out to the parking lot from the oil change bay isn't going to hurt anything.
A word on lower viscosity oils.
The car companies usually designed their engines like this so they could claim a 1% fuel economy boost across an entire fleet, and now you're seeing the squeeze because those are the grades we're running out of first and worst. But don't think you can just pick up some 5w30 and put it in there.
Although Honda and others made plenty of perfectly good engines that used 5w30 for decades, they have designed these newer engines such to where you'll ruin them pretty fast if you find some 5w30 on the shelf and think you'll use it. CAFE really sucks. Nobody on the planet will notice the difference in fuel economy on a 0w16 or 0w8 contribution the oil viscosity makes to the equation. You won't be able to track it even if you consistently use the same gas pump every week because of other variables like where you drove, how many red lights and stop signs, highway or city, was your air filter dirty or clean, what temperature was it out there that day?
Personally if I bought a car that said 0w16 but 5w20 was also allowed and that came without any "in a pinch" verbiage, I'd find a good fully synthetic 5w20 and run it, but only if that's what the engineers who designed it said I could do on a routine basis.
My mom's second husband was a decent mechanic and in the mid 90s to 2000s cars that switched from 10w30 to 5w30, he always recommended 10w30.
I've never encountered a problem with 5w30 even on cars that racked up over 400,000 miles on it. In fact, with a brutal midwest winter ou'll probably increase wear if you use a 10w30 during that season because it takes the oil longer to be pumped up to the upper parts of the engine.
Older conventional oils that were not as refined as the ones you can buy today at a reasonable price were more prone to "shearing" down to a lower viscosity while in the middle of an oil change interval, so if you started with a 10w30, you might shear it down way faster, and that would really be the danger of starting with a 5w30 conventional.
Oil viscosities for an engine have to balance out two different goals. They have to reduce drag and friction so you get ideal lubrication of engine parts, and they also can't be too thin, or you'll get some metal on metal contact and excessive wear. The guidance on the oil cap/owner's manual is almost always best and very rarely do manufacturers come back and issue new guidance.
The most serious case I can think of was when GM had some defective V8 engines on a truck that called for 5w20, and they "fixed it" by changing the specification for 5w40. The oil caps got replaced with ones that say "5w40" and the customer got "re-educated" because it was cheaper than fixing the engine.
@AndrewF 2019 G70 3.3T is what i'm driving currently - bought it with 23k and have just under 41k on it. It's my first experience with RWD, always drive FWD vehicles. Prior to that my VW beetle took 6.3qts of synthetic 5W40 for the 2.5 I5 engine. I would have kept it for a few more yrs it ws at 87k but hit a deer and totaled it. I'm also familiar with the GM 3800 engines - we've had a couple of them in my family (older gen 87 Bonneville followed by a 99 Grand Prix). I drove that 1999 grand prix for another 120 mi after blowing the head gasket. The car wasn't worth more than scrap metal leading up to that as it had a lot of issues and just barely got around town. It had broken tie rod ends, PS leaks, no more overdrive, torque converter was going, everything was so rotted out (was a northern car before it was handed down to me) that you couldn't do anything but toe angle for an alignment.