I couldn't get any of the Castrol TWS 10W-60 locally, the guy a parts store told me to buy Mobil1 'European Car Formula' 0W-40 and said it would be fine to add to the existing TWS 10W-60. I just need to add 1Qt for short term, then I'm getting a full change soon. Is this OK to add? It feels wrong adding something that is not the intended formula, I don't trust the guy and need to crowed source this answer from you guys. What is the census?