is it OK to use regular motor oil instead of synthetic to top off your BMW when it

[DrJohnn]

indicates "low oil"? Will regular 10W 40 oil do any damage when used in lieu of synthetic oil?

 

[Climaticsman]

There is no scientific data to support the idea that mixing mineral and synthetic oils will damage your engine. When switching from a mineral oil to a synthetic, or vice versa, you will potentially leave a small amount of residual oil in the engine. That's perfectly okay because synthetic oil and mineral-based motor oil are, for the most part, compatible with each other

Read more: http://www.carbibles.com/engineoil_bible.html#ixzz1cYjAA9si

However, it may invalidate any warranty if you mix or use the wrong oils