Hi @taedy
You may not be able to use thermal paste on the CPU.
I'm not sure as it isn't mentioned in the [https://download.lenovo.com/consumer/mobiles_pub/v14_and_v15_hmm.pdf|hardware maintenance manual] for the model ''series'', but in the parts list for the model there is a thermal pad listed. (part # 5P20S37559)
Usually thermal pads are used either on the SSD or on the CPU.
If there is a thermal pad on the CPU, then do not use thermal paste as well as they're not designed to be used together.
Go to p.51 to view the procedure to remove the fan and heat sink assembly so that you can access the CPU.
If there is no thermal pad on the CPU then here's the ifixit [guide|744] guide that may help.
As to which one is best to use, everyone has their own preferences.