Learn Spanish (Latin America): Añadir el lubricante al telar diariamente.

In Spanish, the infinitive is frequently used for impersonal, written instructions where no specific person is being addressed. You will see this all the time in manuals, recipes, and on signs. Since this sounds like a line from a maintenance manual, Añadir is the standard way to express this step.

Could I use the verb agregar instead of añadir?

Absolutely. Agregar and añadir both mean "to add" and are largely interchangeable in this context. In fact, agregar is generally more commonly used in spoken Latin American Spanish, though añadir is still widely understood and frequently appears in formal writing.

Why is the preposition a used in al telar instead of en?

The verb añadir usually pairs with the preposition a to indicate the destination or what you are adding something to. Here, you are adding the lubricant to the loom, so a combines with the masculine article el to form the mandatory contraction al.

Is diariamente better to use here than todos los días?