Is topping up the oil enough of an oil change?

Question

If you use synthetic ester-based oil in a relatively new petrol engine, top it up regularly, and the engine runs very quietly and basically without any noise whatsoever (especially after another top up), and can easily go up the minor hills without having to go into a lower gear (with automatic transmission), do you actually still need to do full oil changes at all?

Consider that it doesn't appear to be uncommon or even worrisome for the oil consumption to be around 3k to 6k miles per litre or quart, which means that every 12k to 24k miles, you end up with about 4 litres or quarts of new oil in the engine, leaving only about 2 litres of old oil still in the system after a year or two.

From personal experience, I'm currently running my 2008 Jetta SE 2.5L 5W40 with about 6 litres of top ups (~1L Castrol, 1L Petrosin, then after learning about ester-based oil, 4x ~1L of Red Line Synthetic Oil (or perhaps even 5x)), which basically means that there is almost no original oil left in the system.

The last oil change was done at a dealership in November 2010 at ~29k miles; it's now September 2014 and ~68k miles (68k − 29k miles, ÷ 5,7L, ends up about 7k miles per litre of oil); after the most recent top up a few weeks ago, the engine runs very-very quietly, and I'm still getting excellent mpg after doing these topups, and the engine can currently easily handle modest hills in the normal gear without automatically switching to a lower one up (I've noticed that going up the hills was a problem when the oil was low (at the point of needing topup) and prior to adding the 5th top-up, but it wasn't even a problem when I was about to add my last 6th one up, probably because I'm now all Redline and also it wasn't below the recommended minimum this time around).

I plan to finally do an oil change soon, myself at a DIY shop; is there anything I should look for or be familiar with when trying to do an oil change at such an irregular interval? It'll be 100% Red Line Synthetic Oil, of course. I'll probably buy an OEM oil filter from the dealer.

Answer 1

In a word: No. To add more to it: Absolutely Not.

There is one huge thing which you have not taken into account. That being carbon which deposits from the air/fuel mixture burning process. Where does it go? Right into the oil (among other places). A small amount of blow by occurs which also forces this mixture down into the crank case. Now you have it in the oil. When you don't change it out, it stays indefinitely in the crank case. Some of it gets trapped inside the oil filter. It doesn't sound to me like you've changed the filter, so now you've maxed it out and are circulating all that dirty oil right on through because the bypass has opened up. All that cruddy, dirty oil is running itself through what used to be nice bearings. Now, not so much. Those oil scraper rings are doing their best, but you haven't given them a chance. Just an ugly scene at this point.

Something else you failed to realize is that you still have old oil in your system no matter how much replacement you've put in. It's not linear, but rather like this:

• 4 quarts of oil, with 2 quarts gone, gives you 2 quarts left (Oil1).
• You add two quarts to it, you now have 2q Oil1 & 2q Oil2
• 2q goes away. You now have 1q Oil1 & 1q Oil2.
• Add 2q Oil3, you now have 1q Oil1, 1q Oil2, & 2q Oil3
• 2q goes away. You now have .5q Oil1, .5q Oil2, & 1q Oil3
• Etc, etc, etc.

I hope you get the picture. Even with oil which has an extended life, such as Mobil1 Extended Life, you should still replace the filter more often then you change the oil, because they will not stand up to the extended life. When you replace, you then top off the oil as needed.

While logically it may have the sound of truth to it, realistically you've probably caused irreparable damage to your internals. An engine which may have lasted 200k miles, might now only last 100k. Only time will tell.

Answer 2

Your logic is way off. You should change your oil and filter at the manufacturers recommended intervals, with an oil of the manufacturers specification. The manufacturers cover many many miles under many differant conditions to decide a specification and in most circumstances the specification cannot be bettered for the intended use of the vehicle.

Answer 3

My car - Diesel Golf 2005 Reg has been owned from new by me. I have changed the oil at the variable service intervals when the car has told me to. They are at roughly 18,000 mile intervals and I have used Castrol synthetic oil every time - Currently Castrol Edge 5w - 30. I have done between 20,000 and 37,000 miles a year. The car has now done 317,000 miles and uses 1 litre of oil per 1,700 miles and started to use oil at the 200,000 mark. At this level of usage I don't change the oil at all but change the filter at 18,000 miles. Usage seems to have leveled off at this current level for the last 70,000 miles. I'd say that was pretty good usage for the mileage I've done and would say oil these days is very sophisticated and can last a lot longer. Our perception needs to catch up.