I have a 2005 petrol C4. The car is supposed to have 5W-40 in it at the moment. I have a tiny oil leak which would cost as much as the car to fix, so the cheaper solution is to top-up the oil every now and them.
Yesterday in the store I was browsing the oils and they had "Text your reg to this number to check which oil" so I did and the response suggested I buy the 10W-40. Not thinking about anything I did it and just now it hit me that I'd be mixing different types of oils.
From browsing left and right I come to understand that as long as the second number doesn't change I'd be fine, but not much is written about synthetic and semi-synthetic oil mixtures.
So can I add some semi-synthetic 10W40 to my synthetic 5W40 ?