15a Chemical compound added to diesel to improve its ignition properties
4 LTRS ?A?X I can no thinking its AMYL but the Y mucks up 6d which is DIGITOXIN
28a Variety of swiss cheese 7 LTRS G?U?E?I I am sure there is a swiss cheese similiar to these lettere but so for it has eluded me and the crossword solver has drawn a blank?