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Adding conventional oil to synthetic

I have a 1995 Subaru Legacy (2.2L boxer engine) with about 175,000 miles. The engine has run on full synthetic since new, but is now using about a quart every 1000miles. Last oil change an oil treatment additive was added to the 5w30 Mobil 1 synthetic, but it didn’t help. Since adding synthetic is expensive, can i add conventional oil to the synthetic? Will switching to conventional oil help stop the oil loss? Any risks in making this switch?

Any SAE rated motor oil can be mixed with any other with no problems. That’s one of the requirements to get a SAE rating…You might try a 10/40 oil during the summer months…A quart every 1000 miles is not that bad…

Synthetic and conventional oils can always be mixed. There are no problems associated with this practice. The switch is unlikely to curb oil loss however, but your oil losses for a 15-year-old car is realistic.

You can mix the oils together, or just switch to conventional oil to save money. The engine doesn’t care.

Neither will stop the oil usage. Additives won’t help either. Don’t waste your money.

Check the PCV valve. If it’s faulty it can cause excess oil consumption.

How much is leaking and how much is burning? The 2.2 Subaru engine can leak from quite a few places.

I’d put conventional oil in, one for the older cars, may swell up some seals and reduce use. I’d also use the highest viscosity your manual lists for your car (sometimes there’s a range).

Thanks for your answers. I’ll check the pcv valve.

i’m not sure how much is leaking, i suspect most of it is burn since there’s no signs of leakage under the car. Oil consumptionis monre noticeable on long highway runs. I understand synthetic oil by nature is thinner and may get past the seals easier, but conventional oil should gum up the seals a bit and theoretically help prevent oil loss,… or maybe not?

exactly what i was thinking (see previous answer). Are there any negatives to this (increased fuel consumption, wear, etc)?

This is probably obvious, but just to mention it, if you’re doing long synthetic oil change intervals you either need to top off with synthetic or change the oil at the regular change interval. Since the long change intervals are really the main reason to be using synthetic (in a car that doesn’t require it), I’d add another vote for just switching back to conventional oil at the next change. Adding 3 to 5 quarts of synthetic between changes will get expensive!

Just out of curiousity, what change interval have you been using?

You’ve touched a few subjects. I’ll respond one at a time.

You can mix regular oil and synthetic. They’re both exactly the same chemically. The only differences are that synthetic molecules are more consistant in size and synthetic has fewer impurities. Instead of mixing it yourself, you can even just buy “blended” oil. That’s already mixed for you. has a good primer on oils. I recommend a visit.

Switching will not help stop oil loss, but it also carries no risk with it as long as your owners’ manual does not require synthetic. Using the highest base weight recommended in the owner’s manual might help a wee bit, but don’t have unrealistic expectations.

By the way, a quart every 1000 miles on an engine with 175,000 miles on it is perfectly healthy. As long as you see no other signs of a problem like an operating problem or contaminated coolant, you need do nothing at all but monitor your oil level and add oil when needed.

In summary, I don’t think you have a problem. The engine’s internal condition can be assessed with a simple compression test, but I see nothing in your post to justify even bothering. Your oil usage is not at all unhealty.

yes, synthetic does get expensive. oil changes have been about every 7500 miles, which is what the owners manual calls for (i’ve driven 4000 miles since the last oil change). I hate putting out the money for synthetic when it’s burning off at this rate, so i thought i’d add conventional 10W40 oil to the current 5w30 synthetic until the next oil change, when i’ll go switch to conventional oil.

It’s probably a bit more than a quart every 1000 miles. What i now want to do is just add conventional oil to the current synthetic until the next oil change (see above)

Just checked the owner’s manual, it doesn’t call for synthetic but does warn that SAE 5w30 is not recomended for sustained high speed driving. I have been using 5w30… perhaps that’s why i seem to be burning more on longer highway drives…

I think the compresion is ok. The only other thing i can think of is the PCV valve.

I bet it didn’t come with synthetic, so you mixed it once before…

Just start using conventional oil from now on, it’ll mix perefctly. But try upping the base weight to that recommennded.

NOTE: you can also mix weights without danger. But you may want to start the new weight with a fresh oil change. An extra oil change never hurt anybody.

Yes, you can mix. There is oil already marketed that is a mix.

This is not good to hear that an engine run on full synthetic since new is burning 1 quart per thousand.

At this late stage of the game for that car, I’d try 10W-40 dino (conventional) oil. If the burning stops, good. If it still burns oil, the 10W-40 may eventually damage your cat converter and the problem may be other than piston rings or cylinder bore. 10W-40 can eventually damage the cat converter if the engine burns oil.

10W-40 dino oil will make very cold winter starting more difficult too.

Thanks for your comments.

But I’m a little confused about your comment regarding possible damage to the catalytic converter. If it’s already burning oil, what difference does it make to the cat converter if it’s burning synthetic or conventional oil? Or will a heavier grade oil (10w40) do more damage than a lighter grade (5w30)?

I’d probably change back to a lighter grade for the winter months.