Mixing oils
 

Say I have 10W40 oil that's getting low and I need to add a bit. Can I add only 10W40, or can I add some 5W40 or 10W30? What will be the outcome? Will a mix of 10W40 and 10W30 be something like 10W35? Same for 10W40 and 5W40?

Also, what might happen if I suddenly switch from 10W40 to 5W30 or 0W30? Leaks?

First off, your engine will not blow up if you top off with a different oil. In fact I use 2x 0-w30 quarts and 2x 5-w30 quarts when I change my oil. So mixing them, in my experience is okay.
How old is the car? If the car has a whole bunch of miles on it (180000+), and you have been using 10-w40 for the life of the car, and change to 0-w20, you may loosen up deposits that are holding things together! I had a buddy who got an 80's Cordoba that he changed to lighter weight oil and it flushed so much crap out of the system that it clogged the oil pump and seized the engine. I'm not trying to frighten you, and that should not discourage you from mixing your oils. If I have some 20-w50 lying around, I have used it in the past to top off my car which is supposed to take 5-w30. You, in most circumstances, will not do any harm to your car. Oh, and mixing synthetic with conventional is okay, too. How do you think they get synthetic blends? Good luck.

EDIT: Oh, I just noticed that you have a diesel... 15-w40 is probably what you are running right now. I am not sure if they sell different weight diesel oils, I have never owned one. But I do know that you need to look at the TBN number (9 is great I think, the higher the better). Diesel oils are made to work with an engine differently than petrol engine oil, the diesel oils have more additives. Check out various diesel oils and pick the one that has a good TBN number and is recommended for turbos. Sorry about the confusion.

Thanks, Funny:)
My car is a 4yo turbodiesel with 105k km on it, and I have 5W40, which is the suggested oil (I wrote about 10W40 for easier arithmetic). I've been thinking 5W30 or 0W40, I'm not sure which move is better? Maybe straight to 0W30? Or maybe the half&half approach, so if there is anything to loosen, it'll happen gradually?

Really you are only interested in the second number in the oil weight during the summer. 5W40 means the oil acts like 5 weight oil when cold (20C I believe) and 40 weight oil when its hot (100C). The only reason these multigrade oils exist is to aid starting the engine when its cold and to reduce wear as the thinner oil starts circulating a bit quicker.
If you add xW30 you are thinning your oil at operating temperatures, probably fine if your engine is in good shape, maybe not a good idea if its old and getting more worn out.
In the summer in poland conventional oil 15W40 is fine, during winter is really when you need 5W40 which is usually synthetic. Also make sure you are adding diesel oil, not gas engine oil.
Ian

Thanks for this old thread. Answered my question. So, gonna mix 3 parts 0w30 to 1 part 5w30 when I change the oil tomorrow (just because that's what is laying around and I want to use it rather than buy more).