One of the most famous examples of a split infinitive is in the introduction of the original Star Trek TV series: “to boldly go where no man has gone before”.
The infinitive “to go” has been split by the addition of the adverb “boldly”.
Most split infinitives follow this model – an adverb is put between “to” and the verb. But you also get compound split infinitives where more than one word is put between “to” and the verb, for example: If they make me a partner I can expect my salary to more than double.
So is it correct English to split infinitives?
This has been a subject of debate for many years. Until relatively recently the consensus was that splitting infinitives is not good style. Nowadays, however, most linguists and grammarians accept that users of English do habitually split infinitives and therefore it is not wrong to do so. This comes largely as a result of the change from the strict prescriptive approach to grammar (rules determine usage) to an attitude that, to some extent at least, says usage should determine rules. Also, there are times when your writing will be more fluent and your meaning will be clearer if you do split an infinitive.
Consider the following example¹:
Our monitoring system has failed adequately to provide the required information.
A system can fail partially, or fail completely, but can a system fail adequately? Surely what the writer means to say is this:
Our monitoring system has failed to adequately provide the required information.
And here’s another example²:
He considers it unwise to attempt radically to alter taxes on large cars, as proposed by Labour.
Here we can’t be certain which infinitive the writer is being careful not to split. But the sentence would make much more sense if it was written like this:
He considers it unwise to attempt to radically alter taxes on large cars, as proposed by Labour.
So, my answer is yes – it is OK to split infinitives.
¹ From R.L.Trask, Mind the Gaffe Penguin Books (2001) p. 270
² From Pocket Fowler’s Modern English Usage Oxford (1999) p. 547
Common Mistakes in Business English 
In the first example the word adequately must be replaced by one that makes sense without splitting the infinitive. Grammar has gone to pot. Any educated person, who wants to mae a good impression will rely on the prescriptive grammar. Then, there are those who have no clue.