Fat over Lean in Oil Painting?

Hello art friends! I am about to try oils for the first time. I have been studying a bit on the processes, brushes, mediums etc. If I am understanding the “process of layering” correctly, the first layer should have solvent to thin the paint and the final layer should have no solvent and more medium than previous layers. Solvents and mediums are two different things. I currently have gamsol in my art supplies but have ordered some Lavender brush cleaner which is less toxic than gamsol and I hope I can also use as my solvent to thin initially layers?