It's really simple... use whatever's cheapest and does the job well for what you want.
There are some basic needs that need to be met:
- Remove production grease/oils.
- Remove production/transportation 'gunk' or shavings.
- Apply a corrosion protection product.
If you get those done, you're golden. You'll need one of the many solvents mentioned for #1 and will help with #2, but a brush and elbow grease are also needed. For #3, any type of gun oil is going to keep your investment protected, just some better than others.