Volvo :: Engine Get Warm And Noticed Oil Pressure Light Flicker On And Off

Last weekend I decided my brothers 850 T5 was well over due an Engine oil and AFT change (two years for oil and never for the atf). The car has done 182,000 and drives great. Any way I bought 20 litres of 15w-40 and 20 litres of DEX 3 atf as it worked out a lot cheaper and it can be used for my fathers 940.

Anyway on Saturday I got the engine warm and noticed the oil pressure light flicker on and off. it had never done that before so I should have checked the dipstick there and then but I just went ahead and drained the old oil and removed the filter. To my surprise only about two litres of black oil came out despite the fact my brother swears he checked it 2 weeks before and it was fine. I thought this was odd as the engine had been running perfect and at that level I would expect serious damage to have been done. I fitted the new filter and sump plug with washer, filled a 5 litre tub from the bigger 20L tub and poured in 4L. I checked the dip stick and it took the remaining litre to get the level correct. I checked it again and then started the engine to get the oil into the filter etc and the level was still perfect. I finished the atf change(that's another story) and thought it a job well done.

The next evening my brother was about to use the car and I said I would check the level one more time now it was cold. There was nothing on the dipstick. There was no oil under the car so I slowly topped it up but nothing showed on the stick. I knew there was at least 5 litres in there so I didn't want to over fill it but when I started it, it sounded tappety. I put another couple of litres in and the noise reduced but still nothing on the stick. he then drove the car for a short journey and it drove fine but I said it was still tappety. I checked the oil again and now the handle snaps off the dipstick.

I have done oil changes many times before and am really very competent mechanically (I'm an engineer). Any way this gets me doubting the dipsticks accuracy so I suggest he let our neighbour who's a mechanic take a look the next day and basically today he came back and said he drained the oil and there was only 2.5 litres of oil in the engine. He topped it up and it's running fine and I fixed the dipstick with glue to check and it's perfect there as well. The thing is I know that I put enough in there, and my 20 litre tub has only 10 left.

The car is fine now. I know the obvious ones are that it was my mistake and I didn't put enough oil in, or I don't know the difference between a 1 litre and 5 litre tub but I assure you that this wasn't the case. Is there any where internal this quantity of engine oil can leak to? the transmission? coolant?