I have a 1992 with over 200k, and I haven't gone to synthetic motor oil because of warnings about leaks. The car already has a slight seep from the rear main seal, and a leak there would be a $serious repair.

Then I came across a discourse in the FAQ on motor oil--someone in the discussion said that Castrol Syntec was not a true synthetic but merely a super-refined dino oil.

This correct? If it is, then would it give some of the superior performance of true synthetics while not causing leaks?

Thanks for any help you can provide.

Doug Harvey

I switched my car over at ~250K, the wife's at 190K. She had no leaks. I replaced my seals as part of routine maintenance just prior to switching.

The Mobil 1 formulation has been changed recently to help swell oil seals and prevent the leaky problems that synthetic can expose. True to its word, I can see a reduction in the varnish-like deposits inside my cylinder head, having run the synthetic for ~1.5 years.
--
Speed Racer, '83 240 R, '74 164 E, '93 940 OL1 (Manchester, CT)

If you are worried about leaks, before attempting a switch to synthetic at that milage,youmay want to try a flushing with auto rx ( http://www.auto-rx.com/ )to get the old gunk out and help the seals a bit before trying the switch.
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-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel

In the US, a "synthetic" oil may be made of group III, IV, or V base stocks, while "conventional" oils are made of group I and II base stocks (mostly group II, often with some group III, IV, or V (sometimes labeled "synthetic blend" in this case), for current oils with API SL or ILSAC GF-3 ratings).

The controversy is that group III base stocks are a highly refined mineral oil base stock whose performance characteristics are similar to group IV and V base stocks, which are made "in the lab" (i.e. not from mineral oils). Group III base stocks are much less expensive to manufacture than group IV and V base stocks, but that lower cost is not generally reflected in retail pricing in the US.

Most major brand "synthetic" motor oils sold in the US are made with group III base stock. This includes most or all varieties of Castrol Syntec sold in the US. Of the major brand "synthetic" motor oils sold in the US, Mobil 1 and Mobil Delvac 1 are made of group IV and V base stocks (note: Caterpillar Synthetic Diesel Engine Oil is relabeled Mobil Delvac 1).

Castrol Syntec is probably the most well known of the group III "synthetic" oils because of a legal battle between Castrol and Mobil over whether a group III oil can be labeled as "synthetic" in the US. Castrol won that battle, but that caused some people to especially dislike Castrol for "diluting" the definition of "synthetic" (although most other major brands have also gone to group III base stocks for their "synthetic" oils).

I put a group III "synthetic" oil (Shell Rotella T Synthetic, $13 per gallon at Wal Mart) in an old Volvo with about 167000 miles on it without any new leaks. However, the rear main seal leak (which was leaking "conventional" oil) was fixed not too long before.

This quote from Castrol's site should establish that Castrol sure see it as a synthetic!

"Myth #1: Once I use synthetic motor oil in my car, I cannot switch back to conventional motor oil.

Fact: Even if you've switched to a premium synthetic motor oil like Castrol Syntec, you can always go back to a conventional oil, but why would you want to? Consider Castrol Syntec Full-Synthetic motor oil a "super lubricant," as it seeks out and neutralizes harmful particles in your engine, fights sludge on vital engine parts, neutralizes acids in your engine, and is fully compatible with conventional motor oil. For example, Castrol Syntec actually engulfs particles preventing them from grouping together and forming engine sludge. Synthetic motor oils deliver superior engine protection versus conventional oils, and once you try it, while you can go back, we doubt you'll want to.

(This might also interest you. I cannot verify the hype though:)
Myth #2: Synthetic oil is only for new cars.

Fact: A quality synthetic motor oil can be used in old as well as new cars, including cars in which conventional oil was previously used. While using a synthetic motor oil in newer cars has been well documented, Castrol Syntec also provides added protection through enhanced seal compatibility- which is extremely important for used cars. Therefore, synthetic motor oils are beneficial for the good health, long life, and top performance of new and old cars. And any consumer looking to get the most out of their car should use a premium Synthetic motor oil."
http://www.castrolusa.com/products/productpage1.asp?sc=sdsbl&product_category=1&product_id=4#overview

Erling.
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My 240 Page

Synthetics do not CAUSE leaks. They FIND them. The reason for this is twofold.

1) Synthetics provided better lubrication and can seep into cracks that dino can't.

2) Synthetics clean out gunk/sludge from dino oil. Sometimes this gunk/sludge is sealing up the leaks from the cracks in seals.

Thus, the illusion that synthetics cause leaks exists.

That being said, don't switch to synthetic or a synthetic blend (blends are a waste of money IMHO) because either of them will expose those leaks even more. Fix your leaks.
--
Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'

true syn oils differ from dino and partial syn oils by molecular definition. All offer lub qualities, but the more refined the molecule, the greater realm of lube properies. Bottom line...You have 200k on the car, changing oil types at this stage will not pay off in the end unless you are operating in a climate that is considered severe. Just my two cents.