If I understand this correctly, you want to run a soak test using OpenSTA to identify any memory leaks.
And you want to know which NT performance counters to use to help you identify the memory leak.
The counter I would watch for signs of memory leak would be "available MBytes" and "Pool Nonpaged Bytes". If the amount of available memory reduces as your test runs then there's a sign of a leak. Similarly if you have a rising value of Pool Nonpaged bytes then it's also a sign.