Oil question

[jfreak53]

Ok I have heard to change the viscosity of the oil you use on older higher mileage cars. I have a 1997 GMC Sierra that has 196 on it, the car calls for 5W-30, but I remember someone telling me once to put 10 in it since it's older and higher mileage? What's the thoughts?

I also live in Guatemala Central America right now with it and it's about 70 to 80 every day, not like texas though, it's not scorching, no humidity.


Thanks in advance for any advice.

[MagicRat]

The decision to use thicker oil on high mileage vehicles all depends on oil pressure.
If the pressure is good, especially at hot idle in gear, then there is no need for thicker oil.
Do you have a factory pressure gauge? What does it say? If the pressure is low at hot idel then increases a lot when you rev the engine a bit, then you probably have a worn oilpump and/or excessive bearing clearances due to wear, and thicker oil will help.

If the idle pressure is good with minimal increase as revs increase, then there is no need for thicker oil.

If you are in doubt, install an accurate auxiliary mechanical gauge and see what that reads. A hot idle of 20+ psi is adequate, while 35+ at higher revs is fine.

Finally, 10W 30 is not particularly thick, especially for your climate. Using 10W 30 will not cause any harm. But 5w 30 will give you slightly better gas mileage, simply because it produces slightly less drag on the oil pump, and it lubes slightly better during a cold start.

[jdmccright]

The only difference between 5W-30 and 10W-30 is their viscosity when cold. 5W is easier to pump when cold, thus it gets circulated slightly faster. But once it is hot, both will perform the same as a 30-weight viscosity oil.

In your climate, you should not have to worry about lack of lubrication due to slow circulation from cold oil, so you could probably use 10W-30. Indeed, many truck owner's manuals for that era allow the use of 10W-30 for operating environments that stay above 20°F or so. Check your manual.

For older vehicles, the switch to thicker oil by some is typically to reduce the amount that leaks out from seals when the vehicle is sitting and off. Thicker oil drips out more slowly.

Hope this helps!

[jfreak53]

Great, thanks for all the advise guys. I'm going with 10 probably since it's harder to find 5 anyways ha ha. Thanks a lot.