MTFCA Forum

No, this isn’t a “what kind of oil should I use?” post. I use Pennzoil 10w-30. I have some jugs of 5w-20 which I bought to use in a modern Mercury that I later sold. If I mix the two, will I have 7.5w-25, or doesn’t it work that way?

If they're both the same brand (and probably even if they're not), your theory about mixing them is probably correct. I do it all the time if I end up with some "orphan" oil. It goes into the jug that eventually ends up in the Model T. So far, no harm no foul.

Yes, they both are Pennzoil. I hate to throw out 2 or 3 5-quart jugs of oil

I was told by my dear ole Dad, never mix oil. Why I never asked, I figured that he was older and new why.

I've heard that it's because different brands of oil have possibly incompatible additives that react and cause them to permanently leach out of the oil, leaving a separated portion (up to 1/3) that has no lubricating qualities whatever. I have never confirmed this with a petroleum engineer but that's what I've heard.

my understanding is that all oils rated by the SAE are required to be mixable

A very expert Model T mechanic told me he uses 15W-40. Another, who probably drives more than the rest of us combined, uses Kendall 20W-50. (I don't know why Kendall.) Ford recommended 20W. But 5W-20 is NOT the same as 20W. I would definitely not mix in the 5W-20 at 50/50. Maybe half a quart at each oil change, if any at all.

I don't know what this has to do with anything, but when I was a kid I used drain oil from a fleet of farm tractors and trucks. I drove the T a lot too. I don't think any of the farm vehicles had oil filters and as I recall the oil was VERY black.

extracted this info from this site https://mechanics.stackexchange.com/que ... -viscosity ( was posted on it in NOV 2018)
From the Chevron.com site: "Can I mix different viscosity grades of motor oils?" "Yes. It is always advisable to not mix motor oil brands, however, different viscosity grades of the same brand motor oil are compatible. Be aware that mixing viscosity grades will turn out a product that is different in viscosity than either what was originally in the engine or what was added."
From Shell.ca: "If you mix viscosity grades such as a 5W30 low-viscosity oil and a 10W40 higher-viscosity oil, it is reasonable to expect that the resulting product will have viscosity characteristics which are thicker than the 5W30, but thinner than the 10W40. This change does not reflect incompatibility - it's simply a re-balancing of the viscosity characteristics. In all other ways, the product should work as expected. But there's absolutely no danger about incompatibility resulting from mixing engine oils," Miller says. "We've tested all of our grades and brands, and we haven't observed any problems."
From Mobil1.com: "For our customers to choose a viscosity grade, we recommend they follow the engine manufacturer's recommendations as indicated in their owner's manual. There is no need to mix two Mobil 1 viscosity grades when one will do; however, we see no problem mixing different SAE grades of Mobil 1 Tri-Synthetic series motor oils."

As for a T, I doubt it would care much at all if was mixed, as long as there was some kind oil in the crankcase. Dave

...as long as there was some kind oil in the crankcase.

Or passing through the crankcase on its way to the floor.

Thanks to everyone for your input. I especially like Frank Brandi's post, which seems to have quite a bit of factual and helpful information.

Is there any written proof that Henry said anything other than a light weight high grade machinery oil. In 1955 when many of Ford people of the times were still with us the book Tin Lizzie by Stern said SAE-10. Then as today much depended on where you lived and the temp and season.Oil is cheap,engines not so much!!!! Bud.

Pictures of old oil cans. Draw your own conclusions.

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We need Rich Chillingworth to comment on all this as he is a petrol eingeer....just sayin

I see a post by "Squirrel" above. I recognize you from the HAMB; surprised to see you here.

When you get that all figured out, the can markings, grading and contents are soon all going to change again.

I read someplace that the early engine oil was not much thicker than kerosene or automatic transmission fluid.

That would not work well today, as any drip is frowned on!

No not oil weight : Oil type ! I've read discussion on the forum where using Shell Rotella 15W-40 diesel oil was discussed. Any merit to it?

I use a lot of Rotella 15-40 in many engines but not in our stem wind model T"s Bud.