Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Oil weight question

Our cars both use 5 - 30 weight oil. My husband is wondering if adding about half a quart (a pint) of 40 weight would help the engine run better in the summer. We live in South Texas and it gets brutally hot in the summer.

Use what the manual recommends. Does it list an alternate weight for hot weather? If not, don’t.

And where would he get a pint of 40 weight? Any single-weight oil probably doesn’t meet the oil specifications for many reasons, and could cause problems, rather than prevent them.

+1 for @texases .

+2 for Texases.

The result of blending the 40w with 5-30 can be calculated

but only after many years of experimentation could any determination be made as to the benefits of doing so. What do local dealerships and shops recommend?

+3 for texases.

Modern cars are designed for the oil specified in the manual. My guess is that your cars’ manuals specify 5W-30 and that oil is fine even for your climate. My Toyota Sienna calls for 0W-20 synthetic, no matter where I use the car.
I’ve run the Sienna over the interstate when the temperature was above 95 degrees using the air conditioning of course, and never had the oil go below the full mark.
An air cooled engine used only in the summer is a different issue. I have a 22 year old lawnmower that is now consuming oil–I fog for insects when I mow. I am trying to get one more season of use by using 40 weight oil. It may help a little in this case, but I would never do this in either of my vehicles.

I don’t know that any 40 weight oil that wasn’t specifically designed to be added to a multiweight oil, might not have additives that would dilute the intent of the multiweight in some way. It isn’t like you look at advantages of different oils and make your own concoction . There are too many variables, chemically, to think that long term it would be a car worthy strategy. Unless your hubby is a chemist or an engineer for a car company or oil company, I would disregard his advice.

Years ago (late 1950s through the mid 1960s) there were people, including Tom McCahill, who didn’t believe in multi-viscosity oils. In his book, “What You Should Know About Cars”, published in the early 1960s, he claimed that 10W-30 was a lousy number 10 and a lousy number 30. He used straight weight oil in his cars. In fact, he used one weight higher than recommended. He would use 30 weight in the spring and fall where 20 weight was specified. He claimed that he didn’t have an oil burner in 80-100,000 miles. Well, you can use multi-weight and go at least that far. Most of the wear on an engine occurs when you first start the engine and the oil hasn’t circulated through the bearings and has drained down off the cylinder walls. The heavier weight oil flows more slowly when the oil is cold. Thicker oil means lower flow at start up.

Tom McCahill died in 1975. A lot has changed since he had those ideas. Wth , he can’t defend himself regardless.

A 10w30 might be a better choice…there’s variance in the weights and It might be heavier on the top end, too…an “almost 40” vs a “barely 30” of a 5w30.

Xw20 weights are a touchy subject. Toyota and Ford call for them…in countries subject to CAFE-like testing. Toyota says they provide “maximum economy” and “acceptable wear”; FORD recommends against their use in high-GVWR applications and in their supercharged Mustang.

I would stay away from 20-wt myself, especially in an engine where it has been “back-spec’d” after the fact. The 0.5% MPG hit is worth the peace of mind to me.

I’ve put 10w30 in my car that calls for 5w30 before going on a long road trip in warm weather.
Maybe it doesn’t matter at all in my circumstance, considering the quality of today’s oils, but it makes me feel better.
Toyota did back spec the engine to 5w20.