Learn Spanish (Latin America): La editorial imprimió otro ejemplar, lo cual fue un éxito. - The publisher printed another copy, which was a success.

The relative pronouns que or el cual would refer specifically to the noun ejemplar (the copy). However, the success wasn't the physical copy itself, but rather the entire action of printing it. To refer back to a complete idea or clause rather than a single noun, we use the neuter forms lo cual or lo que.

Could I use lo que instead of lo cual in this sentence?

Yes, they are mostly interchangeable here. Both can refer back to an idea that was just mentioned. Lo cual sounds slightly more formal and is very common in writing, while lo que is standard in everyday speech. Just remember that lo cual can only refer to something already stated in the sentence, which fits perfectly after the comma here.

I thought editorial meant an opinion article. Why does it translate to "publisher"?

This is a tricky noun that changes meaning based on its gender. When it is feminine (la editorial), it means the publishing house or publisher. To refer to an opinion article in a newspaper (an editorial), you must use the masculine form: el editorial.

Why use ejemplar for "copy" instead of copia?