What is "Top-Up" oil???

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Why do we see all these Euro oils (mostly 5/10-30's) being called "Top-Up" oils? What is that? Are they only suitable for "topping up" and not for a complete interval? Is it to dilute or thin the existing oil that has thickened after many mi.'s of use rather than using the original viscosity?
 
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Top-up oils are for cars that are on LongLife Service. I guess, they are a cheaper alternative to the 18 Euro/liter LL oils. Euro manuals also state that 1/2 quart (or so) of certain other oils may be added between oil changes. I think I saw an explanation on a European ARAL site some time ago...
 
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I believe top-up oils have a higher concentration of additives so that in addition to bringing the oil level back up to par, the additives in the old oil are "beefed up" as well. This is particularly important for some of the very long drain intervals that European cars are going to now.
 
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