I believe I know the answer to this...but I'll throw the question out there anyway.
Over time, and the course of oil changes, I've collected a few jugs. I've always liked Castrol, used to use their 10W-40 but can't hardly find it anymore, so I've been using 5W-40. These...
My question, probably obvious: Are there any issues, concerns, drawbacks or compromises associated with mixing these to use in my next (upcoming) oil change?
They're both Castrol synthetic 5W-40 weight oils. What's the difference? What exactly is "European Formula"?