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I read somewhere that when the oil evaporates, the leftover oil is effectively thicker, e.g., to extrapolate, 5W40 may be closer to 5W50 after the evaporation takes place than before so.

I use Redline Oil 5W40 (fully-synthetic ester) in my 2008 Jetta SE 2.5L with 80k miles, which requires oil of type VW 502 00.

Should I still top it up with 5W40, or should I go with 5W30 for top-ups?

I mostly live in the south (Austin, TX), and top-ups are only required past 5k miles (I don't change oil too often, since I use fully-synthetic ester).

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    @Moab, all liquids certainly do; redlineoil.com/product.aspx?pid=3 RedLine 5W40 is supposed be reduced by 6% after 1hr of 250°C. – cnst Aug 8 '16 at 23:06
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    @Moab, I'm not sure about that – you can smell oil, which suggests to me that some fraction of it is becoming airborne. I think that's more or less what evaporation is. It's a whole different question to ask if the rate is significant under normal operating conditions or if it results in a change of the properties of the oil. – dlu Aug 8 '16 at 23:16
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    @Moab, what is the difference between "evaporation" and "cook off?" – dlu Aug 8 '16 at 23:59
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    @Moab from the wiki page you linked: "Liquids that do not evaporate visibly at a given temperature in a given gas (e.g., cooking oil at room temperature) have molecules that do not tend to transfer energy to each other in a pattern sufficient to frequently give a molecule the heat energy necessary to turn into vapor. However, these liquids are evaporating. It is just that the process is much slower and thus significantly less visible." – MooseLucifer Aug 9 '16 at 13:17
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    @dlu Well obviously one is liquid vaporization, and the other is a cooking competition. – I have no idea what I'm doing Aug 10 '16 at 10:00
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Suggest you ask the person who wrote the article. While oil is a liquid and all liquids have an evaporation point, engine oils are designed such that the evaporation point is higher than any temps your engine will ever see.

Bob Is the Oil Guy is a site that has been around forever, and no one knows more about engine oils than Bob: https://bobistheoilguy.com/#

If you use a good brand synthetic oil with a viscosity according to your engine's specs, and change it within the interval specified by the oil maker, you don't have to worry about oil properties beyond that.

  • >If you use a good brand synthetic oil with a viscosity according to your engine's specs, and change it within the interval specified by the oil maker, you don't have to worry about oil properties beyond that. Spot on – Martin May 8 '17 at 9:49
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As multigrade oil ages, the polymeric additive package breaks down, reducing the oil's ability to lubricate across a wide variety of temperatures. A 10w30 oil will eventually revert to the original 10 weight base stock. Utterly useless in warm season operation. No matter what oil you use, it accumulates contaminants, its detergents degrade, and by the time the 3000 mile mark is reached, the engine is being bathed in dirty, compromised oil. Use synthetic oil if you wish, but given its cost, and the necessity of timely oil changes, the costs may outweigh the benefits. A top quality conventional motor oil, with a 3000 mile oil and filter change interval, is your most economical choice for long engine life.

  • Oil analysis proves that synthetic oil changed at 10k miles still has lots of life in it. I changed my oil at about 40k miles once, and it was still fine (including an acceptable TBN of 2.1 after 39.3k miles (with lots of top-ups!), and it was before I fully switched to Redline). – cnst Aug 9 '16 at 21:15
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    @Graybird6 I think you meant 10w30 reverting to the original 30 weight base stock. The pour point and vis package provide good "w" winter flow characteristics, not the other way around. Although oil is pure magic and I could be wrong. Do you have a reference for this statement? I always like to learn tribology stuff, but I'm getting old enough to be a dinosaur. Dig me up in a million years and get a quart of crude out of me... – SteveRacer Aug 10 '16 at 3:44
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    Interesting. My information is dated (I'm a dinosaur too) but it came from Texaco and Quaker State technical service departments. According to them, the base stock was 10 weight, to which was added a package of detergents and long chain polymers. The polymers gave the 10 weight oil, the ability to mimic a summer weight oil. – Greybird6 Aug 11 '16 at 10:12

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