2000 cadillac devilleOctober 27, 2015why would a person change oil for this car from 10/30 sae to 15/40 diesel oil ? Keywords:Deville 1 thought on “2000 cadillac deville” Freeautomechanic October 28, 2015 at 7:58 am It is difficult to answer for the actions of others. Many reasons someone might use 15w40 instead of 10w30. Should it ever be done? No, the engine was not designed for it. 1) They may have only had 15w40 oil on hand. 2) Someone dared them to do it 3) An experiment 4) old wives’ tale – in the old days if an engine started to knock or loose oil pressure some people would add thicker oil as a means to quiet the engine in an effort to prolong its life.. did it or does it work? NO it does not work. Once the damage is done, its done. Comments are closed.