Im fairly certain it is silver base.. I am however very sure it is NOT silicon based.
Inspection of thermal paste should be done on thermal pastes every year (preferably 6 months) (removing a heatsink unevens thermal paste, so you need to re-apply some). After checking the paste, if the consistency is the same as that of fresh off the tube, then cleaning the heatsink/processor and reapplying the thermal paste is critical for maximum performance. If in the first year's (or 6 months) inspection the consistency is good then you can reapply the paste and let it sit for 1 to 1 1/2 years.
Arctic Silver 5 has been known to sometimes last a very long time (in tube/out). I have opened up a couple 3-4 year old computers with AS5, and found their paste to be in the same consistency as fresh off the tube. Results seem to vary however online, anyone else have any ancient AS5 recollections?
i have used most heat sink compound or grease and it is in my opinion all pretty much the same. the key to success is to apply it properly. do not over apply. also smooth any rough non polished surfaces to a smooth surface for better heat transfer. just my own experience. ham radio tech 35 plus years. thanks poorguy