As engine oil gets old, when it's hot does it become thicker or thinner than new oil ?

Thanks.

From a total novices point of view, do the bits of metal in it play any part in it being thicker?

Thinner, it becomes diluted with petrol, the petrol is getting past the piston rings on the compression stroke.

[Edited on 22/4/08 by Mr Whippy]

Hi there are some good bit's of info explained regarding oil on the WSCC forum. Here is a direct link to the basic explenation but there is other more techie part as well. LINK 1 And LINK 2 To the page with the other info.

Cheers Matt

i vote for thinner; from what I recall, the large hydrocarbon molecules get "chopped up mechanically" (oil pump,etc so become smaller.

Can go either way, have seen oil like treacle but then again I've seen oil like water. May depend on the type of oil , the quality , the type of use and how old it is.

The long chain molecules getting chopped will lower it's viscosity.

Heat tends to evaporate the lighter fractions of the oil, making it thicker.

Combustion products entering the oil make it thicker, e.g. sludge formation.

IME the thickening processes tend to win if the engine oil is left long enough.

My nephew's Calibra engine was knackered by cheap oil thickening up. It coated the gauze on his pickup pipe and starved the crank. It was like a layer of tar on everything in there.