I have an old Audi A4 which consumes a lot of oil and has done for years. M OT test fine and no obvious leaks nor particularly smokey. We have been us ing 5w30 fully synthetic oil and to be honest given how long a litre of it lasts (around 400 miles for a litre) I can't imagine whatever we put in it would make much difference.
I have looked online and some seem to say to use this oil and some mineral oil. Also, I was wondering if a thicker one would be better (assume 5w40 is thicker?).
Will changing to a synthetic 5w40 or even to mineral oil make any differenc e?
MOT test fine and no obvious leaks nor particularly smokey. We have been using 5w30 fully synthetic oil and to be honest given how long a litre of i t lasts (around 400 miles for a litre) I can't imagine whatever we put in i t would make much difference.
l oil. Also, I was wondering if a thicker one would be better (assume 5w40 is thicker?).
20/50 is thicker, changing to that can reduce consumption. It's a common tw eak on a worn engine, the cost is a fraction more friction, but not enough to notice any difference at idle on an open loop controlled engine.
A litre per 400 miles isn't good, you may want to consider the other dodge of adding a tin or 2 of that treacley oil to the engine oil. Really reduces oil burn.
There is one gotcha with oil consumption: a diesel that burns oil can go in to runaway & destroy itself.
Yes, but if its not smoking one has to ponder where its all ending up? In the old days when i were a lad, running old engines was the usual way of things on old bangers my father had. Nobody cared about smoking engines then of course. Brian
400 miles per litre is higher than you'd expect these days, although once would have been well within spec for many engines.
Have you checked Ebay etc for a source of the correct oil at the best price? Your garage doesn't pay anything like the 15 quid or so a litre they charge you. By buying in bulk.
400 miles a litre is *not* a badly worn engine. Even more so if it runs OK. Could be something as simple as valve stem seals failed. And using a much thicker oil in an attempt to reduce costs may just result in the engine really wearing out faster.
The car we rented in 1975 or so for a photo safari in Kenya was an Alpha Sud, and very reliable it was, too, given the sort of off-road stuff we were doing.
As you almost suggest, the devil is in the detail.
This is what tabby said earlier
"20/50 is thicker, changing to that *can* reduce consumption. It's a common tweak on a worn engine, the cost is a fraction more friction, but not enough to notice any difference at idle on an open loop controlled engine.
A litre per 400 miles isn't good, you *may* want to consider the other dodge of adding a tin or 2 of that treacley oil to the engine oil. Really reduces oil burn"
and he's right, that was one of the traditional routes from the days where bore and ring wear, or valve stem wear, resulted in significant "burn"
As you say, it *could* be failed valve stem seals. No such thing in the old days, but I've never seen a comparison of how much of the historical improvement in consumption comes from the introduction of seals, and how much from improved materials and engineering of piston rings and bores, plus better anti-wear oil additives.
My academic friends in places like Leicester and Warwick reckoned that one of the big steps forward was the introduction of plateau honing for bores. You arrange for the first machining to introduce deep scores (and high peaks), then you remove the peaks by honing to provide the running surface for the rings, while the scores provide an oil reservoir which gets replenished from below. It's certainly remarkable how you no longer have to "run in" a new engine, or pistons following a rebore like we all did when I first started rebuilding engines in the 1960's.
Thanks all for your help. To answer a few questions and add some clarity.
It is a petrol Audi A4 s-line 1.8 turbo which has done 125k miles so likely the engine is a bit worn. It had always used oil but not to the current ex tent. Besides the cost there is always the hassle of constantly topping it up. When the oil pressure sensor detects you have forgotten to top it up an d you are in the middle of nowhere and you realise you forgot to top up the spare can in the boot. All a bit of a PITA.
As you say we can't know from here what the cause is. I remember one where consumption was traced to oil being dumped on the exhaust manifold. When I say 'traced' I mean it all became clear when it caught fire.
Synthetic oils were developed to extent oil change intervals. If they stopped piston rings bedding in - which is basically wear - they'd also stop other bits of the engine ever wearing out.
The *big* difference with synthetics is that because they don't contain such a variety of molecules as mineral oils, their oxidation performance is better. So they last longer at higher temperatures. Bedding in is influenced by by boundary lubrication. This comes from the anti-wear additives like ZDDP in either mineral or synthetic lubes. Mineral oils often have enhanced boundary lube properties because they contain fatty acids (as impurities) which, if you like, are nature's anti wear additives. Automotive lubricants (whether mineral or synthetic) are fairly highly "engineered" to optimise their properties (because the car manufacturers specify the required performance for a given engine). Further down the market, the effectiveness of things like 3 in 1 is almost certainly partly down to the fatty acids. (This product also contains effective anti-rust additives, which I suspect are responsible for the distinctive smell, although I have never managed to find anyone to confirm that).
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