oils
I have an E30 325. When I bought it, I had no idea what I was doing, and so I took it to Jalbert. Jalbert's the bunk, but that's another story. A long one.
Anyway, they changed my oil to a regular 5W30 mineral oil, since it was the winter. For the next oil change, I used Castrol Syntec 10W30. Great, great oil. The old mineral oil got black in 5000kms. The Castrol is still in there, still looking great, and it's still that nice golden-brown colour. Come to think of it, tho, I should change it.
Change your oil every 4000 to 5000 kms, depending on how you drive (around town, highway, really really fast all the time, etc.). It is the best, best thing you can do for your car.
This is mostly due, admittedly, to a tune-up and new exhaust system (I *HIGHLY* recommend Supersprint, btw), which included a new Lambda (oxygen) sensor, which, in turn improved my air-fuel ratio.
BUT, with the synthetic oil, the chemical properties are such that it has a higher thermal viscosity breakdown threshhold. You can feel it when you take some oil (possibly even from your own dipstick!!! wow!!!!) and press it between your fingers. It'll feel sorta spongy, assuming you've got good oil in there. The sponginess is a primitive way for you, the owner, to meter the condition of your oil. This, and by looking at how black it is. Oil is a clear-goldish colour when it's new. It shouldn't stray too far from that colour, with respects to opacity. It's going to get darker, but by no means should it become black. When it's black, it means you've got too much blow-by soot in there, which could in time damage some delicate and expensive parts (pay attention, you VANOS drivers!)
Also, the synthetic oil has a better operational range for temperatures; it stays more fluid longer, and evaporates much slower than normal mineral oil.
Castrol, Mobil-1 and the like are all excellent brands, and you'll see, you're going to get better performance out of your 328.
