Subarus and synthetic oil

I’ve always used Mobil 1 in all my cars and am convinced it’s the best thing since sliced bread. However, my Forester recently started going through massive amounts of oil, and I ended up having to have the cylinder heads replaced (may or may not be related). Now, I’m still going through oil more quickly than I should. The shop says that they don’t recommend synthetics for Subarus, and that might be part of the problem. I hate to switch back to conventional oil though.

Any thoughts?

Synthetic oil has nothing to do with it. The shop is grasping at straws & who knows why. Presumably the head was replaced for valve issues, but if you’re burning a lot of oil it probably needed new rings. Yet, you didn’t even say what year the thing is, how many miles it has on it, or why - exactly - the heads were replaced. You also didn’t say how much oil you’re actually using. So its hard to say too much - except that the synthetic oil is not relevant.

However, there’s also no reason at all not to go to conventional.

How old is this Subaru (mileage/year)?

Synthetic is not a issue with Subaru however they run just as long with conventional oil.

I will go on to admit I use conventional oil(spec’ed) in my 2005 Legacy turbo that is problem free with the engine/turbo thus far(120k).

1999 Forester with 10400 miles. I completely run out of oil (four and a half quarts) after about two months. I change the oil/filter every 5000 miles.

I was just wondering if anyone else had made the connection. Thanks for your replies.

Do you top it off before you “completely run out of oil”?

I think I know the cause of your high oil usage…your practice of allowing the engine to run completely out of oil before tending to it.

Sadly, at this point you’ve probably allowed pretty accelerated wear on the sleeve bearings (crank & rod), crank, cylinder walls, and rings. Have it assessed if you’d like, which could be as simple as a compression check or as expensive as a check of one of the main bearings, but my guess is that this engine is pretty much history. Learn to monitor your oil routinely and top it off when it gets a qt low (see your owner’s manual). If you’re willing to keep dumping oil into it, and you don;t mind changing the cat converter and the upstream oxygen sensor every now and then, you might get more years of life out of the engine, but your high oil usage is not affordable to correct. It’d require a new motor…or a rebuild.

By the way, even though the oil light may not have illuminated, that does not mean you haven’t severely compromised your engine’s life. The oil washes your cylinders and removes heat. The amount of oil determines how well it can do it’s job. It’s like washing the kitchen floor, you can do a nice job with a full bucket of water, but you cannot get the floor clean with 12 oz in the pail. You’re in essence trying to keep the insides of your cylinders clean and cool with very little of the necessary fluid.


Thanks for your thoughtful reply, I never let the oil completely run out (horrors!) it just gets very very low and I need to keep an eye on it. Tired of topping it off though. Just wondering about the synthetic angle.

Synthteic oil is chemically the same as dino oil. Its differences are that its molecules are allegedly more consistant in size and that it contains fewer impurities. Because it contains fewer impurities it stands up to heat better, and for the same reason it allegedly retains its lubricating properties better in extreme temperatures. It’s that durability that causes it to be recommended for extreme use aplications, like in a turbocharged engine. Turbochargers operate at extremely high rpms (200,000) and are heated by the hot exhaust…right next to where the bearings ae lubricated by the oil.

Synthetic does not cause problems in engines that don’t require it. Dino oil can cause damage in engines that require synthetic. Because they’re chemically exactly the same, they can be switched back and forth and even blended without risk in engine that don’t require it. You can even buy it blended off the shelf.

“Horrors” is an excellent description of running out. But if you think about what the oil has to accomplish, it’s not inaccurate for running very very low either.

All the best. Life can be a challange.

Thanks for your thoughtful reply, I never let the oil completely run out (horrors!) it just gets very very low and I need to keep an eye on it.

You should NEVER EVER let it get below the fill mark…very very low tells me you’re doing just that. I have no idea why anyone would do that to their vehicle.

The object of the game is to never allow the level of the motor oil to fall more than 1 qt below the full mark. Personally, I add 1/2 qt as soon as my dipstick shows a 1/2 qt drop, even though my engine has a 7 qt oil capacity. The OP’s engine probably has a 5 qt capacity, which means that it is imperative to keep the oil level full or very close to full at all times.

Once you allow the oil to fall more than 1 qt below the full mark, the reduced quantity of oil is called upon to attempt to provide the lubrication and cooling of the full amount of oil, but it is unable to do so. The result is that oil consumption then increases geometrically.

Then, when the oil is down…let’s say…2 qts, this geometric increase in oil consumption increases at an accelerated rate. This is a steady, ongoing process of increased consumption that quickly results in GREATLY increased engine wear. As the engine wear increases, the oil consumption increases even more, until you get to the point where the OP is currently.

In essence, based on all available evidence, I believe that the OP wore out his/her engine by failing to monitor the oil level and allowing it to get “very, very low”. That is why your Owner’s Manual instructs you to check the oil every time that you fill the gas tank!

At this point, the type of oil that you use is almost irrelevant, due to the vastly increased wear in the engine.

Sorry for the bad news. Hopefully you will treat your next car better than you have done with this one!