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Can you use a charger with a higher wattage than your laptop?
Yes, you can safely use a laptop charger with a higher wattage as the laptop will only ever draw the amount of power required at the time. If your laptop was supplied with a 35W charger it will never draw more than 35W unless being fast-charged, so you can use a 65W, 90W or higher charger. To reassure you even further, you can safely charge your 5W AirPods with a 240W charger if you wish.
If the charger has multiple ports that extra juice will mean you can charge other devices at the same time.
Can you use a charger with a lower wattage than your laptop?
You can but you should try to avoid using a lower wattage charger than what your laptop requires. A 30W charger can’t supply sufficient power for a 90W laptop’s needs. At the least, your laptop is likely to run out of power while you are using it. At the worst, you could damage the laptop or create dangerous problems for the charger.
The under-powered charger won’t have enough power to give to your laptop and it therefore has to work harder to try to match the needs of your laptop. The danger is that this can cause overheating and potentially fire.
For more details read our explainer What wattage USB-C charger can you use to charge a MacBook?
I swear I've googled this and been through 5 pages of results and haven't gotten anything except ads for specific chargers or pages about fast charging. Specifically, I'm looking for a car adapter to stick my MagSafe 3 cord into (so USB-C) but I need it this weekend and the only ones I can find at local shops are 45W. I don't need fast charging, I just need it to charge. Would that work or am I a bit screwed?
Just 96W as the other charger.
It is not the charger that decides how much power a computer is charged with. The computers draws the power from the charger - and the MacBook Pro 16-inch (2019) will only draw 96 watts. That goes even when the charger is capable of delivering more power.
You might instead want to look into conserving power. For example it could be that you have programs unnecessarily running and consuming CPU, or it could be that an update exists for some of your software that is more energy efficient.
You can also try for example lowering the brightness on the internal display to save power.
The power rating of the adaptor is the maximum it can supply. It's not a hose that always pushes that much juice through the pipe.
The device takes what it can use. If Apple made a 500W adaptor, your Mac wouldn't get 500W.