hi all, i've been baffled by the new formulation from royal purple 15w40.. I noticed that there is a 'HD' label attached to this oil.. as in the case of liquimoly... the other grades of RP (eg: 10w30, 10w40) do not have the 'HD' label some suggest that HD indicates 'for diesel only'. my '99 sentra runs on petrol. can I use the above grades for my oil change??? thanks!!