Most stable conformation of aldehydes and ketones
I found in Ian Fleming's Molecular orbitals and Organic Chemical Reactions, page 93-94 the answer (the aldehyde is propanal):


  The polarisation of the H-C bonds in the methyl group leaves a weak
  positive charge on the outside and this is attracted to the partial
  negative charge on the oxygen atom, lowering the energy of this
  conformation


(the difference in energy of the two eclipsed conformations of propanal is 8 kJ/mol).