Les implies that being relaxed about the use of split infinitives is a modern development. The truth is - as forcefully recognised by "the original Fowlers" back in the 1920s - split infinitives as such have always been perfectly acceptable English. The point is to avoid using them in such a way as to lose clarity or read clumsily. And there is no difference between American English and English English in this respect.

"A real split infinitive, though not desirable in itself, is preferable to either of two things, to real ambiguity and to patent artificiality."

(And HW Fowler himself all those years ago wasn't too impressed with "the Victorian grammarians" either, especially the ones who thought that the rules worked out for Latin grammar should apply to English, which is where the fetish about split infinitives has its origins.)