I just use lint free toilet paper. You don't really need chemicals. If the old paste is all hard and crusty you can use a wash cloth with a tiny drop of hot water.
Any rubbing alcohol is ok. Just use a small amount. 2-3 drops, the purity or mixture will not matter, as long as you don't submerge the cpu-gpu and you don't get anything on the contacts
I have used the little alcohol pads that one might use to clean their skin before a shot. Those work well and are clean and easy.