Viscosity

Society of Automotive Engineers "W" system chart for oil viscosity

Fill up at Mill Valley B-100
$4.15 / gallon
mileage 182070

Its a cold end of March for the Eastern Seaboard, but despite the chill in the air my duties called me to drive to the DC area and spend the day outside with a video camera.

Madeline has been leaving puddles of oil behind her front passenger side tire, and last time she was in to see Dr. Doug he said that she'd at one point run nearly dry of oil.  She takes a special diesel oil, 15W40.  So today started with a quest for 15W 40 oil here in Baltimore.  Checked first at the Royal Farms in Hampden, where the face of everyone standing in line at the cash register snapped in my direction when I came through the door.  You get the feeling that place is the nexus of Hampden's street life.  The Farm Store only had 5W30 and 10W40.

Next, I "excuse me sir"-d the elderly gentleman fixing a screen in the back of Sirkis Hardware.  Same deal,  only 10W40 and 5-something.  Up the street at the Mieneke up the street, the girl at the cash register clued me that i'd have have to go to Advanced Auto Parts at 20th and Howard.

At Advanced Auto Parts, a young black clerk with a fanciful neck tattoo was very helpful.  Turned out that 15W40 is only available in gallon jugs-- $15, please.  I fear that I may actually need that much!  Out in the parking lot, I popped the hood and took stock.  I'd put a quart in a few days ago but thought she might need more.  When I removed the cap to the crank case, there was a steam/smoke emanating from it, and I muttered aloud "That can't be good", which attracted the attention of a large, friendly black man.  I fired up the engine, and we looked at it, determining that it was a head gasket issue.  I didn't add any oil after all, since the dipstick showed "full".  And the big friendly guy assured me i'd have no problem driving to DC and back before getting it fixed.  Then he asked if I was interested in selling the car.  I explained that she was a special case who ran on biodiesel, and that I was documenting my experiences with her.  He looked at me quizzically and walked away.

A little research on why diesel engines need 15W40 revealed that that designation refers to starting temperatures in Centigrade, the number followed by "W" referring to the starting temperature in Winter and the other one referring to starting temperature when its... warmer.  And all that time I thought the W meant "weight".

Turns out the viscosity of various types of oils varies by temperature, and the Society of Automotive Engineers created the "W" system to refer to oils by temperature range.