The Chicago Manual of Style is one of the seminal style guides in the world. If you’ve got a grammatical question or are pondering an obscure rule governing the use of the English language (like whether or not to capitalise the E in english, or the names of birds in a tourism brochure) then you should check it out. Especially interesting is the FAQ/Q&A section.
On split infinitives:
“CMOS has not, since the thirteenth edition (1983), frowned on the split infinitive. The fifteenth edition now suggests, to take one example, allowing split infinitives when an intervening adverb is used for emphasis (see paragraphs 5.106 and 5.160). In this day and age, it seems, an injunction against splitting infinitives is one of those shibboleths whose only reason for survival is to give increased meaning to the lives of those who can both identify by name a discrete grammatical, syntactic, or orthographic entity and notice when that entity has been somehow besmirched. Many such shibboleths—the en dash, for example—are worthy of being held onto… euphony or emphasis or clarity or all three can be improved by splitting the infinitive in certain situations. It’s one of the advantages of a language with two-word infinitives.”
Cop that one grammar nazis…