I agree w/ Shipo. All can have an opinion, but we won't be able to resolve this definitively. Oil used for automobiles is very thin at 200F operating temps, and differences in viscosity are unlikely to affect leak rate.

Source of leaks is usually defective gaskets that have lost ability to flex (too hard), leaving gaps from cold to hot temps.

good luck

 

I'm not sure where you got the 7.45 to 10.95 cSt number for xW-30 grade oil; per the chart on the page you linked, grade 30 oils range between 9.3 and 12.49. The above said, the real world difference of a "light" 20 (i.e. 5.6 cSt) and a "heavy" 40 (i.e. 16.29 cSt) is utterly irrelevant when it comes to oil leakage past a seal or gasket. Think about it this way, when cool (i.e. oil at 40°C or 104°F) a typical 0W-20 has a cSt of about 45, and a 10W-40 will typically be above 100 cSt.

Long story short, if an engine leaks past a crank seal when running on 0W-20, it will still leak from the same spot when a 10W-40 oil is used.

 

Oops, you're right .. I read the chart too fast. so 9.3 - 12.49 ... that's still quite a range.

Here's another report that mentions the flow rates associated with the different viscosity grades: http://www.ideas4ag-ed.com/uploads/3..._viscosity.pdf

These rates are measured at temp (210F) and are measured in seconds.

I don't understand how you can assume that oils of different viscosities will flow or drip at the same rate, when these tests and the grading system is based on those differences?

So back to my original question -- was the dino oil the OP switched back to thicker than the Castrol HM? And which 30 weight oils are thicker than other 30 weight oils?

If the flow rate for a 30 weight oil ranges between 25 and 34 seconds, then assuming the oil will continue to leak out of the bad seal, that still might mean a difference of 30% less leakage over time. And if the leakage is on the border of dripping vs not dripping, it could effectively stop the dripping. Will it fix the seal? No ... but it will keep the oil from seeping out, and eliminate a lot of drip marks in the driveway.

Eventually I will have to get the rear main seal replaced (or sell the car before I do), but in the meantime if the drip can be held at bay through the choice of a slightly thicker oil that would be very a very inexpensive alternative.

Fyi, we've had this rear main seal drip every since we bought the car, two years ago. I've only used Castrol GTX HM in the car since. And the dripping has come and gone ... so it definitely seems like it's on the threshold where a thicker oil (or a dirtier oil) can make a difference.

 

The reason why different grades of oil leak past seals at a virtually identical rate is that the difference between say 8 cSt and 14 cSt (typical xW-20 and xW-40) is extremely minor compared to say oil at 100 cSt (coolish 10W-40). Said another way, engine oil at operating temperature will find a way to leak if it has a chance.

Couple the above with over 40 years of experience (I used to think as you do, a higher grade of oil will leak less) which has taught me the sad truth, if you have a leak, no change in grade will help.

 

My experience also.

 

Guess what --

I switched from Gastrol GTX High Mileage to Penzoil dino 5w-30 ... and the dripping has stopped.

We have 1500 miles on this oil change, the engine still appears to use oil, but I haven't seen a drip under the car since. A close look at the bottom of the tranny where the oil use to seep, still shows a bit wet, but no drips forming.

So far so good.

 

^^^ Thanks, jkowtko, for running your own test, which happens to confirm the surprising findings in 1st post.

 

OK - now two (2) users have seen "High Mileage" rated oil initiating oil leaks in one case, and causing oil leaks to continue in second case.
Unknown if Castrol brand is sole problem, but both here were using Castrol High Mileage Oil.



In 1st case, my case, the oil leaks have not returned and I have no drips, now 4,000 miles later.
In 2nd case, jkowtko, has 1500 miles on change away from "High Mileage" oil and the prior ongoing problem leaks have disappeared.

In these two cases, the only evidence we have, OIL LEAKS DO NOT CONTINUE IF SWITCH AWAY
FROM "HIGH MILEAGE" OIL! (CASTROL BRAND, ... but others? No data here.) 'Tis a very nice result.

THERE IS EVIDENCE HERE THAT "HIGH MILEAGE" OIL CAUSES OIL LEAKS IN AT LEAST SOME ENGINES.

 

It may simply be the comparison of those two brands ... Castrol GTX HM being slighty on the thinner, slipperier side ... and Penzoil dino being thicker and more viscous. And of course whatever dino oil UhOh used (can you identify it for us?)

No idea if other brands would compare the same way, or that we can generalize between HM and dino oils here. But I will recommend this as a suggested first step for anyone experiencing the same issue.

 

A friend of mine put high mileage oil in his Escalade and the motor started smoking. Coincidence? I don't think so, but can I prove it without a doubt? No. Not sure of the brand.

I haven't used a high-mileage oil and will never use anything other than your typical motor oil. The only thing I might do is go up in viscosity (say from 5w or 10W-30 to 10W-40) when there are a lot of miles on the motor and the weather is warm.