# Oil in car **URL:** \ **Category:** Factual Questions\ **Created:** [October 3, 2001, 7:30pm UTC](https://boards.straightdope.com/t/oil-in-car/85233 "2001-10-03T19:30:10Z")\ **Posts on this page:** 10\ **Page:** 2
**Author:** ![brad\_d](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@brad\_d](https://boards.straightdope.com/u/brad_d)\ **Post date:** [October 5, 2001, 4:46am UTC](https://boards.straightdope.com/t/oil-in-car/85233/21 "2001-10-05T04:46:58Z")
Thanks, **Padeye**. Having educated myself a bit more on the details of positive displacement pumps, it’s pretty obvious. With a PD pump, there’s never a fluid connection between the low and high pressure sides, so additional hydrostatic pressure on the inlet side can’t be transmitted through to the outlet. On, say, a centrifugal pump, it can. Thinking about it a little more, even if the additional hydrostatic pressure _did_ get carried through, I doubt it would matter much. Oil’s density is, what, about 6 pounds/gallon? Hydrostatic pressure in a liquid is density_gravity_depth, so the pressure increase from high oil levels would be: (6 lbm/gal) \* (9.81 m/s[sup]2[/sup]) = something in horrid units, which I can convert on my calculator to: 2.6x10[sup]-2[/sup] psi/inch. An oil pan filled **1 foot** too deep would have a pressure at the inlet only 0.3 psi higher than normal. I can’t believe that this would be enough to damage anything in the first place. So, yeah, the high-oil-pressure notion seems like bunk, for more than one reason. ---
**Author:** ![cw](https://avatars.discourse-cdn.com/v4/letter/c/71c47a/32.png) [@cw](https://boards.straightdope.com/u/cw)\ **Post date:** [October 5, 2001, 5:19am UTC](https://boards.straightdope.com/t/oil-in-car/85233/22 "2001-10-05T05:19:42Z")
> [@](#): > > \*Originally posted by longhair75 \* > \*\*Good evening, > > Longhair to the rescue! I shut her engine off, and drained out all the oil, replaced the filter, put in new oil, and she was set to go. > > Three years later, we were married. In May, we will celebrate the 30th anniversary of that oil change. \*\* Why can’t something as simple as this happen to me?! Damnit. Sweet story, longhair. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [October 5, 2001, 12:54pm UTC](https://boards.straightdope.com/t/oil-in-car/85233/23 "2001-10-05T12:54:32Z")
> [@](#): > > \*Originally posted by Moe \* > \*\*Thanks all for allowing me to hijack in my own questions. So as I understand it, it is not recommended for me to use higher numbers to slow down the rate at which my engine burns oil? \*\* Common sense would _seem_ to concluded that, all else being equal, thinner oil will burn away faster than thicker oil. But I’ve heard this is not true from a certain “industry experts.” In fact, one claims that using thicker oil will actually _increase_ oil consumption. Wish I could find the straight dope on this… ---
**Author:** ![Una\_Persson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/una_persson/32/346_2.png) [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)\ **Post date:** [October 5, 2001, 1:02pm UTC](https://boards.straightdope.com/t/oil-in-car/85233/24 "2001-10-05T13:02:38Z")
> [@](#): > > \*Originally posted by Crafter\_Man \* > **3. Don’t worry about brands. Just look for a cheap or “on-sale” oil with an API rating of SH or SJ (or better).** The API service category SH oil designation was made obsolete in 1996. If you see a bottle that has “SH” as its highest rating, it could be from a back-stock of oil, and could be pretty old. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [October 5, 2001, 1:20pm UTC](https://boards.straightdope.com/t/oil-in-car/85233/25 "2001-10-05T13:20:51Z")
> [@](#): > > \*Originally posted by Anthracite \* > \*\* > > > [@](#): > > > > \*Originally posted by Crafter\_Man \* > > **3. Don’t worry about brands. Just look for a cheap or “on-sale” oil with an API rating of SH or SJ (or better).** > > The API service category SH oil designation was made obsolete in 1996. If you see a bottle that has “SH” as its highest rating, it could be from a back-stock of oil, and could be pretty old. \*\* Is there an expiration date on oil? And is there a significant difference/improvement between SH and SJ? I’m certainly not an expert, but I doubt any damage would result from using SH. But I could be wrong… Anyway, if someone offered me some “old” SH oil at $0.50 a quart, I would buy it up (and use it) in a heartbeat. ---
**Author:** ![armedmonkey](https://avatars.discourse-cdn.com/v4/letter/a/d6d6ee/32.png) [@armedmonkey](https://boards.straightdope.com/u/armedmonkey)\ **Post date:** [October 5, 2001, 2:38pm UTC](https://boards.straightdope.com/t/oil-in-car/85233/26 "2001-10-05T14:38:41Z")
> [@](#): > > \*Originally posted by brad\_d \* > So, yeah, the high-oil-pressure notion seems like bunk, for more than one reason. \*\* Yeah, I apologize for throwing that out in the first place. It was the way I learned it, but apparently I was taught wrong. Oh well, sometimes fighting your own ignorance means that you have to _un_learn something. Thanks all. ---
**Author:** ![Rocketeer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rocketeer/32/4437_2.png) [@Rocketeer](https://boards.straightdope.com/u/Rocketeer)\ **Post date:** [October 5, 2001, 2:47pm UTC](https://boards.straightdope.com/t/oil-in-car/85233/27 "2001-10-05T14:47:12Z")
**brad d** , I think your math is off. Most oils have a density (specific weight, actually) of about 0.03 lb/in3. So a column of oil one foot (12 inches) high would exert a pressure of 12\*.03 psi, or 0.36 psi, at its base. Incidentally, notice how, using the English system of measurement, we didn’t have to multiply by g? Pretty cool, huh? ---
**Author:** ![brad\_d](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@brad\_d](https://boards.straightdope.com/u/brad_d)\ **Post date:** [October 5, 2001, 9:29pm UTC](https://boards.straightdope.com/t/oil-in-car/85233/28 "2001-10-05T21:29:02Z")
> [@](#): > > \*Originally posted by Rocketeer \* > \*\ ***brad d** , I think your math is off. Most oils have a density (specific weight, actually) of about 0.03 lb/in3. > \*\* Fair enough. 🙂 I pulled 6 lbm/gal out of my head, without bothering to look it up. About the only number I can ever remember is the density of water, which is approximately 8 lbm/gal. Knew oil was somewhat less, but was feeling lazy. 0.03 lbm/in[sup]3[/sup] is about 6.93 lbm/gal. What’s 15% among friends? 😃 English units are evil, I agree. Unfortunately, the rule-of-thumb values that I can recall are often in that system. Bummer… ---
**Author:** ![handy](https://avatars.discourse-cdn.com/v4/letter/h/b5a626/32.png) [@handy](https://boards.straightdope.com/u/handy)\ **Post date:** [October 5, 2001, 10:28pm UTC](https://boards.straightdope.com/t/oil-in-car/85233/29 "2001-10-05T22:28:07Z")
leander, how do you know you put too much oil in? Did you look at the stick to see how much it has? ---
**Author:** ![Una\_Persson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/una_persson/32/346_2.png) [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)\ **Post date:** [October 5, 2001, 11:28pm UTC](https://boards.straightdope.com/t/oil-in-car/85233/30 "2001-10-05T23:28:09Z")
> [@](#): > > \*Originally posted by Crafter\_Man \* > \*\*Is there an expiration date on oil? And is there a significant difference/improvement between SH and SJ? > > I’m certainly not an expert, but I doubt any damage would result from using SH. But I could be wrong… > > Anyway, if someone offered me some “old” SH oil at $0.50 a quart, I would buy it up (and use it) in a heartbeat. \*\* As far as performance improvement of SH versus SJ, I doubt that there is any measureable difference to anyone. IIRC, oil can “expire”, but I have never seen an actual date. I _do_ know that some of the additives will break down over time, and I have verified that 12-year old oil, still in it’s sealed, airtight plastic container can develop a significant, disturbing fractionalization (I saw this in two quarts I examined in an old garage). So…to be safe, I would not use old oil myself. If it was 5 years or under…it likely is not bad. Ten years? Who knows…but then again, I’m one of those people who “wastes their money” by using full synthetic oil for my 3000-mile changes… [Previous page](https://boards.straightdope.com/t/oil-in-car/85233.md?page=1)