VW oil mystery

My T5 TDI 2.5 nearly always needs an oil top-up about a month before it's annual service, I know this because it annoys me to be putting good oil in only to be flushed away a few weeks later. It's done this for at least 10yrs, we do a modest mileage in it and it's still 'only' done about 130k which isn't much for these (our T4 had over 300k when we sold it).
About 2wks ago the oil light came on, this is after about 3k miles since service/MOT, on checking the dip stick was metal just a smear of what looked like old oil at the very bottom (below the low/high marks). My initial thought was that the garage forgot to change the oil or top up, they are adamant they did (perhaps a little too adamant, we all make mistakes!). They've looked it over and no leaks, there's noting on the drive and it doesn't seem to be smoking.
Any ideas? Does the oil change reset the service interval?

VW aren't as uber reliable as people often think.

This isn't unheard of. Used to have an Audi A3 Quattro Sportbrake-with the then top of range 190 BHP TDI engine.Nice fast car.

Burnt oil faster then yours. Constantly having to have it checked. Went back to Audi a few times (VAG) but got the feeling they knew of the issue and just kicked the tyres. Some years latter read that the engines can burn a fair bit of oil- well known apparently.

I'd say give it a new oil and filter change. And two weeks later do it again- but put in some thicker specified oil- see how that goes for a month.

More cost and bother-but unless its smoking oil while driving (best seen at night from rear view) or its pooling oil on the ground- you will have done as much as you can do.

Some engines just drink heavily. Hopefully no harm done to yours.

Regards

Thanks, I had a colleague who was forever putting oil in his Audi TT when I had the 16vt which never drank any which annoyed him as his was much newer. The puzzling thing is why now, we had just done a 250mile round trip at some motorway speeds which might have contributed but pre Covid this would have been nothing out of the ordinary. It's running fine so can't see any harm done, will keep monitoring.

Fun fact is my Alfa GT had NEVER had to be topped up between services in my 12+ years of ownership. There’s no rhyme or reason to it!

Neither of my diesels ever needed a top-up, even at a combined mileage in excess of 400k miles... I speculate, but do not know, that perhaps this is because diesels traditionally run lean by design; wide open air and inject the amount of fuel required, rather than trying to go stoichiometric. This would mean that there is less vacuum (!!) in the cylinders on the intake stroke, and a lessened tendency to leak air past either the rings or the valve stem seals.

Whether this is true in the brave new world of diesels with cats I don't know.

I consulted a friend today, he works exclusively with VW / Audi products. He had the following comments.

If it were the later 2.0 bi turbo the news could be very bleak. The CFCA 2.0 180hp engines have had a really bad reputation of high oil use and engine failure, all to do with corrosion in the EGR assembly, apparently.

The 2.5 though is regarded, by him, as "bomb proof." If there is high oil use, and you are using the correct oil and can't find oil leaking out anywhere, then it could be one of the following,

Oil can leak into the power steering system, but if you've lost enough to bring on the oil light this should have been obvious, with oil coming out of the steering reservoir.

Oil can also leak from the oil cooler into the water system, but it's usually the opposite symptom of water in the oil. Again he said that if this had happened it should also be obvious with oil in the header tank.

A blocked turbo oil return hose or turbo seals. Oil will usually be found inside the intake hoses. This is important to check because excessive oil could cause the motor to "Runaway" basically run on the oil rather than diesel.

And finally - it could be a worn motor, if all other routes look fine have a compression test undertaken.

OR - The Garage just didn't put enough oil in