If what you want is a quotation or offer, then sending a letter of intent makes you look ignorant while requesting a quotation will generally at least make you look like you know what it is that you want. And what you want, after all, is a price quote. Hence you want to request a price quote, via a request for quotation.
"Buyer who is serious, ready and able to purchase [crude oil], will sign [an] LOI and all the necessary documents that protect the rights of the Brokers and proceed. There is nothing to lose in signing those documents. This is how it is usually done and this is how it should be."