On Sat, 04 Mar 2006 08:56:01 GMT, "jw 1111"
> wrote:

>Hi, My car (toyota petrol 1.3) does a smallish mileage in a year say
>around 5000 miles, mainly
>on fairly short distances around town. what would be the 'time' limit to
>reasonably extend oil changes for economy reasons using a semi synthetic oil
>?
>
>I suspect a lot of people will immediately say don't economize in this way.
>However some forty years ago I had a conversation with a professor of an
>engineering college and he said engine oil does not degrade 'time wise' as
>fast as most people think ( and importantly as the oil companies would wish
>us to
>believe). So by how much time could one reasonably delay an oil change with
>modern oils?

Your problem has to do with oil contamination not mileage or
oxidation. If you don't get the oil up to engine temp ( somewhat over
the boiling point of water for the oil, but I've never measured it ),
condensates will accumulate in the oil. Gasoline, water vapor, who
knows what from crankcase fumes. Pretty soon, if you never let your
engine run long enough to boil off these volatile condensates, you've
got acidic crap pumping through your engine that is going to dilute
the lubricity and other wear-resistant characteristics of the oil,
which will lead to early engine death.

So you have to change your oil just as often as anyone else, but for
the contamination reason. You'll have pea soup in the oil sump
eventually, or worse, gunk that will clog the minute oil galleries
that lube the piston bearings and piston pin ( I call it the pin...the
rod that the head pivots on ).

Say at least every 6 months, every 4 months would be better.

Otoh, I'm using full synthetic, and am going by miles not time...5,000
miles change filter and add oil to bring up to full mark on oil stick
( say just less than a full quart for the filter ). And at 10K miles
a complete oil and filter swap.

For me, that means a new filter and somewhat less than a full quart at
25,250 miles, regardless of how long it takes me to get to that
mileage. Then at 30,250, I change ALL the oil and put on a new filter
again.

YMMV
HTH

Lg