My wife has a T420 and I have a W510 which will soon be replaced by a W520... I have an extra W510 135 watt adapter. Would it be all right for this to be used with the T420?
The W510 adapter can be used with the T420.
Using an AC adapter with a higher wattage, as long as it has the same voltage and polarity on the tip (which it does in this case), will not cause damage to the notebook or adapter. However, using an adapter with a lower wattage than the minimum recommended could lead to problems.
For example 65w or 90w adapters should not be used on W510 or W520 systems - they will reduce system performance, may extend time required to charge the battery and could exhibited higher operating temperatures of the adapter.