Old charger from T61 is a 90 watt p/n: 92P1109. Both are 20V and have vrtually identical plugs.
Can the 90watt, 20V AC adapter from a T61 be used on a T510?
ThinkPad adapters can generally be used interchangably so long as the voltage and plug style are matched and the adapter used has at least as much wattage capability as the system using it demand.
In this particular caseT510 can use either 65 w, 90 w or 135 watt Thinkpad charger used on W510.
Systems with discrete graphics often require the higher wattage adapters. The higher wattage adapters may enable somewhat faster battery charging while the system is running. Use of 65w adapters on some systems may result in processor speed scaling, especially if the system is run with the battery removed.