it is best practice to replace the old paste with new...the old paste has been on there since day 1, subjected to temps for prolonged periods..it's going to dry out. Most of the guys on here use and recommend arctic silver 5 for these type of things (and the iFixit site sells it because they reckon it's good too!) reduces temps by around 5c if you check their site..got to be good for your mac's longevity. btw..we don't get paid, so no, this is not a plug for us to get a bonus!