I bought a '92 525iT several months ago. The quick change oil place sticker on the windshield read 20w50 so I stuck with that when I changed oil. Older BMWs I've owned have usually specified that weight for mild climates and/or warm weather. The car always idled rough especially when cold and I chalked that up to fouled plugs or bad cold start valve.
Only belatedly did I check the Bentley Manual to find that 20w50 is not listed on the viscosity chart, only 5w30 for cold climates and 15w40 for everything else. I dumped the Havoline 50 wt and replaced it with Rotella 15w40 and the difference is amazing...even my wife could tell the difference immediately. I can only surmise that the heavier oil was not allowing the hydraulic lifters to function properly especially when cold. Who would have thought that going from 50 wt. to 40 would make such a noticable difference? Or that a 220k mile engine would be so sensitive to oil viscosity?
Only belatedly did I check the Bentley Manual to find that 20w50 is not listed on the viscosity chart, only 5w30 for cold climates and 15w40 for everything else. I dumped the Havoline 50 wt and replaced it with Rotella 15w40 and the difference is amazing...even my wife could tell the difference immediately. I can only surmise that the heavier oil was not allowing the hydraulic lifters to function properly especially when cold. Who would have thought that going from 50 wt. to 40 would make such a noticable difference? Or that a 220k mile engine would be so sensitive to oil viscosity?
