The phrasal verb "top off" simply does not apply to or collate with temperature. You cannot "top off the temperature", the source text is full of holes. Why try and translate an expression that does not exist in English?

It would make sense if it said something like:
"is used to regulate / balance / limit / control the oil temperature prior to delivery......."

So I'd say something like:
"Se usa el calentador para la regulación fina de la temp. del aceite antes de...."
"Se usa el calentador para controlar / regular / afinar / equilibrar la temp. del aceite antes de...."