What causes oil to thicken up?

[Catterman]

When I had my oil analysis done, after 13,000 miles my 5w30 had thickened to a low 40 weight. I am just wonering what caused that? Thanks!

It was a 1995 Accord 2.2 auto with Amsoil ASL & eao20 filter.

 

[Pablo]

In the case of Amsoil's earlier formulas it's mostly the very light base oils being consumed first. Some people think it was oxidative thickening, but it seems to level off and hold in the 13-14 cSt range.

 

[Doug Hillary]

Hi,
Catterman - Many factors can cause it but typically the most common cause of thickening is oxidation (however you like to describe it)

The ACEA test protocols have a "stay in grade" test of 30 cycles at 100C that must be met. Some engine makers have an extended "stay in grade" test out to 200hrs

Staying in grade (not thickening) is extremely important when the engine's design has hydraulic valve actuators and phasers etc.

Previous design diesel engines had an allowable 40% increase (typically due to soot uptake) or a 15% decrease in viscosity before the oil was condemned

ACEAs stay in grade test is an extremely important one as is the more demanding engine manufacturers (Porsche, Mercedes Benz, VW-Audi etc)test regimes. With today's engine designs a thinning of the lubricant by one point (say and SAE40 to an SAE30) within a programmed OCI is better than any thickening at all

 

[buster]

Quote:
is extremely important when the engine's design has hydraulic valve actuators and phasers etc.


I've heard this is one reason Mobil 1 0w-40 should be used.