Being a regular on this board for some time now, I have noticed MANY MANY posts that often revolve around, "how long should the engine oil drain interval be?" And often, the people asking are from the "old school" of thought, where synthetic oil was unheard of, and oil volume was 5 quarts or less, or about 5 litres or less, thus ALWAYS thinking 3-5k miles should the absolute maximum.

Then, when they get advice from those more in the know regarding synthetics, stating that mileages of 10-12K are fine, or about 15-18k km, and EVEN MORE (as long as the oil filters are changed at recommended intervals) if a high quality synthetic made to withstand extreme conditions, as well as having VERY long drain intervals designed into the oil, primarily through high quality synthetic base stock, is used, i.e. AMSOIL.

I have always thought about why my extremely hard driven, and modified Volvo 855T has NEVER, EVER worn through it's AMSOIL, even after a year of use, in Toronto, doing over 15k miles or about 24k km (I had the oil analyzed). You see, engine oils used in areas where there are extremes in weather face differing problems. First off, coupled with the oil cooled turbocharger, and summer heat of 30+ degrees Celcius (easily 90 degrees farenheit) often break down engine oil through high temperature, while cold winter driving in usually (it's been a VERY mild winter this year) -20 degrees celcius and EVEN LESS, often produces a lot of condensation, as well as sulphur, and sodium in the engine oil, as well as having a MUCH higher sulphur content in the gasoline, simply, being north of the border.

Besides having a VERY good engine oil, arguably the best available commercially in AMSOIL, I have often wondered why, the Volvo also has a higher then normal recommended drain interval.

And after recently purchasing a 98' Nissan Maxima SE 5 spd, and finding its recommended drain interval to be significantly less, it occured to me. There is a *hitload of engine oil in the Volvo, especially Turbos, compared to the Maxima. My 855T has a total volume of about 7 quarts of oil, compared to the 4 in the Maxima. Yes, only about 6 quarts is dumped at each oil change, unless someone bothers to do the oil change RIGHT, and dump the oil cooler as well. But that's still a TONNE of new engine oil at every oil change.

But does more oil mean, longer drain intervals? If you look at recommended drain intervals of cars in the same class (I'm talking class as in forced induction, as well as MSRP, because forced induction cars usually have higher stressed engines, and MSRP, because higher class cars generally ask for synthetic, while lower end cars do not), it is pretty darn accurate to say, the more oil in the car, the longer the drain interval.

And when you think about it, isn't it obvious? The more oil in the engine, the more detergents, and base stock, and just plain everything, that must be consumed/oxidized/changed chemically. As this was just a working hypothesis, I thought I'd swing it by some experts. As such, I brought this to the attention of a Formula SAE technician, responsible for engine oil research on a Formula SAE race team. I never produced any of my hypothesis to him, just to insure I would not bias his answer. But as I expected, he concurred. When I asked him why engines with larger oil capacities have longer drain intervals, he gave that exact answer. Just to make sure it wasn't a fluke, I tried to contact representatives from AMSOIL, Mobil, and Castrol. First of all, I must say, niether Mobil, nor Castrol was able to produce an answer. When contacted, each said, they did not know, and simply stated that auto manufacturers had done extensive testing to determine proper drain intervals for their cars, and their drivers, and had essentially left themselves out of the equation in drain intervals (in retrospect, I should have also contacted Quaker state, with their engine oils stating sometimes longer the recommended drain intervals, depending on the car). Only AMSOIL replied with a REAL answer. And as expected, it was exactly what I had thought was the answer, and what the Formula SAE tech had said.

To make a long story short, YES 850 owners, your, YES YOUR, Volvo can have a significantly longer drain interval then what you are used to, IF you use a HIGH quality synthetic engine oil. Why? Because there is SO MUCH oil in your engine compared to 90% of the cars on the road today.

Thank you all for your time. And I hope I have put some of those "long drain interval doubts" or as I like to call them LDID's to rest.

P.S. The next time anyone else tries to get a hard (technical) answer from Castrol or Mobil, be prepared to wait if it does not involve, why their product is good, or where to buy. 'Cause otherwise, be prepared to be persistent, and patient. On the otherhand, within about 2-3 days, I had an engineer provide me with an answer from AMSOIL. I know it really sounds like I'm pushing AMSOIL here, but I really am not, this is exactly how things turned out.