Al-Megrahi - the only man convicted of the 1988 airline bombing that killed 270 people - was released from a Scottish prison last August on compassionate grounds after doctors said he was near death.
But nearly one year on, the alleged former Libyan intelligence agent remains alive, reviving outrage on both sides of the Atlantic over his release.
Sheinwald's comments came as Hillary Clinton, the US secretary of state, said she would look into claims by a group of senators that oil company BP had lobbied the British government to release al-Megrahi to protect commercial interests in Libya.
BP signed an exploration agreement with Libya in May 2007, the same month Britain and Libya signed a memorandum of understanding that led to al-Megrahi's release.
The oil company has denied the allegations, admitting that it did lobby the government over a Libyan prisoner transfer agreement but did not enter discussions on al-Megrahi.
Sheinwald, in an open letter to John Kerry, a US senator, published on Friday, also defended BP from the claims.
Earlier this week, four Democratic senators called for an inquiry, after reports that a cancer expert who backed the three-month prognosis now believed Megrahi could live for 10 or 20 years.
But professor Karol Sikora said on Thursday that his words were taken out of context, and that the chances of Megrahi surviving for a decade were "less than one per cent".
"There was a greater than 50 per cent chance in my opinion that he would die within the first three months then gradually as you go along the chances get less and less," the Press Association quoted him as saying.
"So the chances of living 10 years is less than one per cent, something like that."
A spokesman for the Scottish government said on Friday that it had no contact from BP in relation to al-Megrahi.
"The issues being raised in the United States at present regarding BP refer to the prisoner transfer agreement (PTA) negotiated by the governments of the UK and Libya, and therefore have nothing to do with the decision on compassionate release which is a totally different process.
"We were always totally opposed to the prisoner transfer agreement negotiated between the UK and Libyan governments.
"The memorandum that led to the PTA was agreed without our knowledge and against our wishes.
"The Justice Secretary rejected the application from Libya under the PTA specifically on the basis that the US government and families of victims in the US had been led to believe that such a prisoner transfer would not be possible for anyone convicted of the Lockerbie atrocity.
"[Al-Megrahi] was sent home to die according to the due process of Scots Law, based on the medical report of the Scottish Prison Service Director of Health and Care, and the recommendations of the Parole Board and Prison Governor - all of which have been published by the Scottish Government."