Even in a good environment, the thermal paste is always shot after 5 years - with cleaning being worse on some machines but easier on others. It will still make the job much easier even though you don’t have one of the difficult systems.
Since this is a old Lenovo, you can get away with using high concentration isopropyl alcohol or even denatured if you have it. I don’t recommend the AS5 iFixit sells as I have had trouble with the paste drying out and curing wrong, but have not had such issues with Artic MX-4.
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