I am wondering if it is safe to use a MacBook charger on my Lenovo laptop. Both devices charge via USB C PD. When looking at the voltages I can see that the Lenovo adapter is 20V - 3.25A
The MacBook charger has 20.2V - 4.3A
I am wondering if the .2 increase in voltage with the MacBook charger should be cause for major concern.
The extra 0.2 volts is not a cause for concern. The way USB-PD over USB-C works is that the power supply and device will negotiate a mutually compatible voltage before power is delivered. The power brick will not provide power until the laptop asks for it, and it will only supply a voltage that the laptop is capable of accepting.
If you look closely at the Apple power supply you will see it also lists 9 volts and 5.2 volts output. This is to maintain compatibility with iPads and iPhones that use lower voltages. If the Lenovo can't handle the 20.2 volts then it will step down to 9 volts at up to 3 amps. That's 27 watts instead of 87 watts but its better than a dead battery, dead from not enough power or too much.
The reason they specify the extra 0.2 volts is to account for voltage drop on the cable. At the power brick end of the wire it will be 20.2 volts but at the laptop end it could be 20.0 volts, 19.8 volts, or whatever happens to be lost to heat in the resistance of the wire.
First thing is that the laptop is unlikely to be so fragile to be damaged by such a small variation in voltage. Second, the laptop and power brick are smart enough to not burn themselves up. These aren't like the "dumb" power bricks in the past, they talk to each other to find out what the other is capable of before power is put on the wire.
Oh, and a third thing, Apple and Lenovo make good stuff and they stand behind their products. The USB group will crack down on them hard if they violate the USB-PD spec to the point something is permanently damaged. There was a lot of sketchy USB-C devices out there when it first came out but big name companies with lots to lose worked hard to clean that up. Big names like Apple, Lenovo, Microsoft, Google, and Amazon. Don't buy stuff with "too good to be true" prices and you should be fine.
If everything is USB, then theoretically it will be fine. If there is some incompatibility then it should just not work without damaging anything. That being said, in practice compatibility issues aren't unusual so anything is possible. If you want to be safe you should stick to the manufacturer's recommended charger.
Regarding the 0.2V difference, that's tiny and if there is a problem that's not likely to be it.
I would be cautious when dealing with non-OEM chargers. Since we don't know the internal circuitry, the Lenovo laptop may only be designed to take 20V and nothing more, since overvoltage can damage electronics. That said, I don't think that a 0.2V increase would damage the laptop. Realistically, it may just not accept the 20.2V if it finds a fault.
