My E30 daily driver needs an oil change again, and I now have a nice warm place and a pit to make it downright pleasant to change the engine oil, so one of our brilliant techs did that today. She did some reading up on what the correct weight is and reminded me of the importance of choosing the right oil.

For example, if the factory specifies 15W50 weight for this engine, this means it’s a multi-grade oil with the “15W” being the cold-start temperature.

The lower the number before the “W”, the more runny the oil is at a cold temperature. At a cold temperature, “runny” is good for that number.

The higher the number before the “W”, the less runny the oil is when it’s cold. So, a too-high number means that when you first start your engine, the engine oil pressure might be high enough to damage things and yet the engine internals aren’t being lubricated with oil until later when the engine warms up, and until then you have something that’s more like honey or molasses as to viscocity and so your internal engine parts are wearing very rapidly.

I often park my E30 outside and sometimes it gets single-digits cold here, as in colder than 10 degrees Fahrenheit, so I’d be unwise to go buy engine oil with a higher number before the “W” such as 20W50. If anything, I should go lower on the first numbers, such as “10W”.

As to the number after the “W”, that’s how runny the oil is once the engine warms up. So, at a given temperature, oil with a “50” after the “W” will be less runny than oil with a “40” after the “W”. In that context, too-runny is not good, so a higher number is good.

I would prefer to read the official BMW opinion as to what grade oil I should use, and I haven’t seen that yet, so hearsay and common sense are my fall-backs. On those premises, 15W50 does sound about right. If anything, because I live in the cold North and it IS the middle of winter, I could even go lower and still be OK.

I went to Autozone and tried to find 15W50. They didn’t have any, but they did have 10W40. Those are slightly lower numbers and that’s OK since it is colder weather now. So, that’s what I chose, along with a Bosch engine oil filter.

The oil drain plug is a bolt with a 14mm head, and there is also a washer as a gasket between that and the pan.