You should use whatever voltage memory is stated in the specifications for your system. If it allows 1.35V memory usage, this is better. I believe, however, that most 2nd generation processors were paired with only 1.5V memory.
In the specification that I cited in the question(from intel), there is no mention of voltage. I too think it should be 1.5V but wanted a reliable answer. And also what happens if I use 1.35 V instead, assuming 1.5 is the specification?
Crucial and OWC RAM recommendations for the model list both kinds of voltages as doable!