Viscosity Choice (5w30 vs 10w30)

[txmatt]

I recently purchased a Honda Generator and noticed something in the documentation that I've seen before. The recommended oil is 4-stroke automotive detergent oil with API SJ certification. The ambient temp/viscosity table can be summarized as follows:

5w30: 30 degrees F and below
10w30: -5 degrees F and above
30w: 50 degress F and above

This has never made sense to me. Why the restiction of 5w30 to temps below 30? It acts just like 10w30 at operating temps. It seems like with the recommendation for 10w30 for for -5 degress F and above, 5w30 should be -20 degress F and up or somesuch.

The only thing I've seen on these boards is that 10w30 is a little more stable over the long run due to the lower viscosity spread.

I thought maybe the 10w30 recommendation was because the 10w would leave a thicker coating on parts during storage but when you shut off the engine, 5w30 and 10w30 will be about the same thickness and would seemingly slough off parts similarly until the engine cools.

What gives?

Matt

 

[Bror Jace]

txmatt, 5W30 is stable at most common automotive temps but put it in a higher-temp application (say, an air-cooled engine) and many brands will shear out of grade in a hurry. Even in automotive engines many 5W30s will shear down to a 5W20 in 2,000 miles.

10W30 is usually much more stable at elevated temps (although brand disparity is significant) and of course straight 30 is even more so.

I like the way Honda still gives the owner/user a choice of oil weights for the different temps he may encounter rathee than merely recommending a one-size-fits-all grade.

--- Bror Jace

 

[rpn453]

If you look at older owner's manuals for cars (1980's and earlier) it will usually show a chart recommending different grades for certain temperature ranges. In these, 5w30 is usually only recommended for up to about 10C (50F). It made sense, and still does, to use an oil that is best suited for temperature range, but with the higher quality of modern oils an oil with a greater viscosity spread such as a 5w30 can handle pretty much any temps you'll encounter while driving.
But, as Bror Jace said, a 10w30 is inherently more stable at high temps (when comparing similar base stocks) than a 5w30, and maybe that engine tends to run quite hot. A high quality 5w30 may be better at high temps than some 10w30's, but generally the 10w30 will be better. Even when manufacturers began recommending 5w30 year-round, the turbo engines used 10w30 exclusively.
So I'd stick with the manufacturer's recommendation and use a 10w30, or possibly even a straight 30 if it's always used in warm temps.

[ September 30, 2003, 02:32 PM: Message edited by: rpn453 ]

 

[txmatt]

Thanks guys. I've only run the generator a few hours and was going to do an oil change with conventional oil and put at least a few more hours on it before going to Mobil 1 10w30. I'll heed your advice and go get conventional 10w30 to refill for some more break-in time.

Matt