slowlier or more slowly?

Could you please tell me which variant is correct or more widely-used in spoken English - slowlier or more slowly. I know that according to the rules of forming the comparative degree of adjectives, we should say slowlier, but is it possible to say more slowly and will it be a mistake? I'm quite unsure of a number of similar adjectives like friendly, lovely and so on.
Thanks.

Slowly is an adverb.
More slowly is correct.
Rocstar

I agree. Slowlier is not correct. It should be more slow or more slowly.

Hi Driven:
Slow is a short adjective, so it should be: Slower.
Rocstar

Of course you're right rocstar. Slower or more slowly. My bad.

Thanks a lot, i had so many doubts about those.
The Grammarway3 textbook which we use as a supplementary grammar says that adverbs obey practically the same rules of comparison as adjectives, namely that 1- or 2- syllable adjectives and adverbs ending in -y form the comparative degree by changing it into IER. That's the point. But, to my ear (though i'm not much of a perfect english user), slowlier just doesn't sound all right. What about such words as angry - angrier or more angry? friendlly?

Also pretty? Should i say "she's prettier than my sister" or "she's more pretty than my sister"? Now that i have your answers in favor of "more slowly", i'm completely at a lost. Are we using a bad grammar or what?

Example from the textbook: The children got noisy. The teacher got angry. -- The noisier the children got, the angrier the teacher got. Does it seem correct to you native speakers?

Zukerman. Your sentences from your grammar textbook are correct. Adjectives ending in y take the -ier form in the comparative and -iest in the superlative. So noisy, noisier, noisiest. Happy, happier, happiest.
With your original question the comparative of SLOW is SLOWER, superlative SLOWEST.
Slowly is an adverb and therefore you can use MORE slowly and MOST slowly.
http://www.ucl.ac.uk/internet-grammar/adverbs/adverbs.htm
Have a look at this page for a more detailed explanation of adjectives and adverbs in comparative and superlative form.

Could you please tell me which variant is correct or more widely-used in spoken English - slowlier or more slowly. I know that according to the rules of forming the comparative degree of adjectives, we should say slowlier, but is it possible to say more slowly and will it be a mistake? I'm quite unsure of a number of similar adjectives like friendly, lovely and so on.
Thanks.

Hi Zuk. The real problem comes when you try to turn an adjective

into an adverb. We would only ever say [friendlily] or [lovelily] by mistake or for comic effect. 'Correct' usage would require in a friendly fashion, in a most lovely way, or some other similar paraphrase

Thanks for the address, Paul. It really helped. And I found some useful exercises there.

it's quite easy to understand: all the adverbs that end in -LY make their comparative using the word "more". e.g. slow - slowly - more slowly; quiet - quietly - more quietly
In spite of this, the monosyllabic adverbs like hard or fast make their comparative adding -er.
I hope it helped!