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first thanks for the feedback! Basically I cannot say my engine is burning oil. With the current 10W40 I'm using (Mobil Super S), I have topped up about 2liters in 13,000km (which is about 2qt in 8000 miles). As far as I remember I had slightly more with the previous 5W40 I used but not much.
Can I assume that the consumption is linear? If so, a quart every 6500k/km isn't bad. At that consumption rate, what was your oil change interval. You appear to be attempting to go to 15kkm, while you appear to be quite close to that with 13kkm.
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The age factor for me concerns the wider internal tolerances of the engine due to wear. 5w40 is good for a new engine but it may be too thin in a "worn" engine.
I don't know how old your engine is. I know the mileage. Again, a quart/liter every 6500km doesn't indicate a "worn" engine to me.
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Since both are 40 grades, I thought it was more due to the lower viscosity when cold of the 5w40 oil and thus that the engine was mostly burning oil when cold. However it may also be due to different oil quality (more evaporation etc). No clue about that but I assumed I had more oil consumption with the 5w40.
Cold states are not where I would expect consumption to occur. If consumption can be altered, it may be altered with viscosity ..but all of these oils (including the Mobil 10w-40) are within a narrow enough range for hot viscosity, that I don't think there would be a change in consumption between them if that was the intended outcome.
We don't know the condition of your current oil after your done with it. I imagine UOA is expensive over there, but it would tell you if the oil sheared or otherwise degraded over your typical usage.
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Do you think that the super low friction 10w40 can hold 15k km (9k miles) OCI safely??
If you were in America, I wouldn't hesitate telling someone that they could use a 10w-40 for 9000 mile OCI if they were a 12,000 mile annual driver. On the other hand, I don't know if you do routine Alpine hill climbs for fun ..and don't think anything about them
For, our often required, "peace of mind", I would go with the 4100 5w-40 that's specifically blended for extended drains. I wouldn't react at the first onset of consumption if it's a little premature. It will probably subside as you move forward.
I'd really prefer you go with the cheapest and use it for the full term of 15000km and then do a UOA. That way you can have the best economies. One OCI isn't going to alter long term wear, especially in this narrow range of oils, imo.