Mixing Engine Oil

Ken Krawford · February 19, 2009, 05:06:06 AM

I changed the oil on my engine a couple of months ago and didn't have enough for a complete fill. Sometimes I will use 30 weight and other times 10W30 depending on what a given store has available. Several days ago I went to buy another quart to complete the fill and realized I couldn't remember if I used 30 weight or 10W30.
I always thought it was bad to mix different weights of motor oil so I emailed Universal to explain my problem. Their response was "No real issue here. Add 10W/30 or a W30. They will mix."

What are your thoughts?

Jim Hardesty · February 19, 2009, 06:35:46 AM

My thought for what its worth. Mixing brands or weights of oils wont make make a difference, in the short term. Find out what works best for you and you have the most confidence in. Change that only if necessary. Then change back when convenient. If all the little things are taken care of just right our engines should last decades. Just my 2bits.

Ken Juul · February 19, 2009, 06:51:29 AM

I always thought it was the brand that mattered not the weight of the oil. Unique additives by manufacturer was the reason. Probably BS. Like everything else, I'd guess most refineries make it for several different brands.

Michael Shaner · February 19, 2009, 07:06:02 AM

I always subscribed to the theory of never mixing weights...unless absolutely necessary. Some of the wrong thing in there certainly has to be better than none of the right thing

Jon Schneider · February 19, 2009, 08:39:27 AM

Mixing oils is fine, although Ken raises an interesting point about additives. The one thing you must not mix is standard diesel oil versus synthetic diesel oil. Regular oil and synthetics don't mix. I'm not sure how bad the consequence is of mixing, but it's definitely a no-no.

Hawk · February 19, 2009, 01:44:09 PM

Up here the standard seems to be 15W40 diesal oil for our engines. Some brands actually say "Diesal 10W40" on the container. The key is that it has the CF4, CH4 etc. designation I believe.
I put 10W30 in once and was told it wouldn't harm the engine but I might see some oil leaking. After a week I couldn't stand the uncertainty and so went down to change the oil again.

Any thoughts?

Hawk

Stu Jackson · February 19, 2009, 02:03:21 PM

Quote from: Hawk on February 19, 2009, 01:44:09 PM....and was told it wouldn't harm the engine but I might see...

Hawk, that kinda "quote" is what drives me up the wall. Who told you? What "credentials" did he or she have? What backup to this unsubstantiated rumor did it come from? If you get information from this board or others, you can at least quote us by name.

I, personally, wouldn't listen that kind of potential nonsense and get concerned about it without doing some more checking (OK, I get it, you came here, good on you!).

10W30 is just fine stuff. I use 30W. Synthetic Mobil 1.

Michael Shaner · February 19, 2009, 06:27:15 PM

After Stu's post, it seems appropriate to bring up the synthetic vs. conventional oil question...

If you change it every 100 hours, is synthetic worth it?

Hawk · February 19, 2009, 06:39:11 PM

Stu,
It was the local marine engine shop at the marina who told me and a highly regarded diesal engine mechanic (unrelated to the shop) who recommended the 10W40 as did the PO and others. Maybe its another quirky Canadian thing.........

I'll check into it further and report back. I'm sure that it likely doesn't matter at all. These diesals probably run fine with vegetable oil in them :)

Hawk

Ron Hill · February 19, 2009, 07:11:57 PM

Ken : I've uesd Mobil 1 for well over 40 years in my autos and over 20 yrs in my C34. The advantage of a synthetic over a conventional oil is that a synthetic doesn't "wear out" it gets dirty. Conventional oil does both. Getting close to 100hrs and the end of the season and using a synthetic? Put on a new oil filter and go till haulout!

I've talked to the Mobil 1 engineers and asked about mixing two 10W30 with two 15W50 and the engineer said " You're trying to make 10W40, and I replied - yes". He said that was a great way to do it and there would be no problem!

I quote from a Mobil 1 bottle "fully compatable with engine seals and conventional oils".
I'd say .... Dah , but that wouldn't be polite. A few thoughts.

Stu Jackson · February 19, 2009, 07:15:48 PM

Quote from: Hawk on February 19, 2009, 06:39:11 PM
Stu,
It was the local marine engine shop at the marina who told me and a highly regarded diesal engine mechanic (unrelated to the shop) who recommended...

Nice to know where it came from. He's probably more trustworthy than I am, eh?

Hawk · February 20, 2009, 12:19:28 AM

Sorry, not 10W, I meant 15W40. As for trustworthy, I'd wager you have him beat, Stu.
I'm still checking on the oil specs...........

Michael Shaner · February 20, 2009, 06:05:55 AM

Thanks for straightening me out Ron!

Jeff Kaplan · February 20, 2009, 08:09:56 AM

the standard oil used in all major trucking companies for there macks, cummins, dd, cat and i/h motors is 15w40. it is multi weight,or viscosity, so it thins down to 10w in cold and thickens up to 40w in summer. i only use straight 30w here in n.e. as it is such a short season and we don't need multi vis oil here. mixing oils is ok but never syn. with regular and there is a difference with manufactures additives. buy one type of oil and stick with it. spring is coming. i just attended the boston boat show and saw the 375 catalina. what a boat...jeff

Stu Jackson · February 20, 2009, 10:05:25 AM

Jeff, thanks for your post, don't know if you've seen this one about the 375: http://c34.org/bbs/index.php/topic,4232.0.html