Under heavy load, the 87W adapter (actually true for the 96W as well) may not provide enough power to charge the battery so you may occasionally see the battery level drop even if the computer is connected to power. The 87W adapter will not damage the computer in any way, however.
Apple ships MBP 16" with a 96W charger. If you're just charging the battery, the 85W is fine (just slower to charge). If you're looking to run the MBP 16" off plugin power (more like a desktop environment), I'd recommend using the 96W charger.
Apple is selling the 96W charger as a separate item now: