Oil Question

[Islandwhiteknight]

My 1997 Sierra SLE 7.4/454 dually calls for 5W30. I've been using conventional oil and it purrs. I always have the Big-Bigfoot camper on it and it comes in at 12,000lbs when I cross the scale. It has 260k/160 miles. I use 75w90 in front diff and 75w140 in the rear diff. Minimal mileage so I oil change yearly.
Question: Conventional oil seems to be getting harder to find and after reviewing lots of posts I'm now of the opinion that the newer synthetics might not cause any leaks. If I switch to synthetic, which I understand is thinner than conventional, should I move up to 10w30 given that I'm always carrying a heavy load. I look forward to the feedback. Thanks.

 

[someotherguy]

I wouldn't change grades, regardless of conventional vs. synthetic. Run what the manual says to run in grade, based on operating conditions.

Richard

 

[OutlawDrifter]

10w30 isn't heavier per say...its a touch heavier when cold, but once the engine is up to operating temp, no difference.

Synthetic blend is everywhere if you're worried about running full synthetic. I run Rotella T6 full synthetic 5w40 in my Vortec 7.4L. When I was daily driving it, it liked to sip the 5w30 too much. It now consumes 1qt in 5k miles, probably half of that is leaking out the front main.

 

[454cid]

As others have indicated, 10w30 should perform the same as 5w30 at operating temperature. The difference is cold start up.

Also, why run such heavy rear end oil? I'm not sure what GM says about that but factory fill is either 75/90 synthetic or 80/90 conventional.

 

[rebelyell]

Perhaps the oil clearances in main, rod & cam bearings has increased along with the wear of mileage.

 

[Islandwhiteknight]

As others have indicated, 10w30 should perform the same as 5w30 at operating temperature. The difference is cold start up.

Also, why run such heavy rear end oil? I'm not sure what GM says about that but factory fill is either 75/90 synthetic or 80/90 conventional.

When I first got the unit I learned that a K3500 HD had the option for the heavier oil. Since I was running with such a heavy load I figured it was the safest thing to do and easier on the diff under load.

 

[GoToGuy]

It's not " thinner" , it just maintains it's same viscosity over a wider temp range. That's one advantage of synthetics.
Is your model a K3500 or an actual K3500 HD, is the HD in the model ID?
Where did you find references to using other than 80/90 diff gear lube. The service manual only requires more frequent checks and shorter interval for lube change, if using for constant heavy loads or towing. . No mention of using 140.

 

[someotherguy]

When I first got the unit I learned that a K3500 HD had the option for the heavier oil. Since I was running with such a heavy load I figured it was the safest thing to do and easier on the diff under load.

It's not " thinner" , it just maintains it's same viscosity over a wider temp range. That's one advantage of synthetics.
Is your model a K3500 or an actual K3500 HD, is the HD in the model ID?
Where did you find references to using other than 80/90 diff gear lube. The service manual only requires more frequent checks and shorter interval for lube change, if using for constant heavy loads or towing. . No mention of using 140.

Exactly on the differential - while the window sticker may have included the words "heavy duty" this is not a 3500HD in the sense of that note in the manual saying 75W140 synthetic gear lube is used. That is for the Dana 80 rear axle used in the 3500HD chassis cab trucks. Chassis-wise they have nothing in common with the lighter GMT400's.

Your K3500 should be using 80W90 GL-5 in the differentials, per the manual.

Richard

 

[Orpedcrow]

I’ve never seen any evidence that synthetics ever CAUSE leaks. It’s usually a far superior oil anyway. I really like redline.

 

[df2x4]

Your K3500 should be using 80W90 GL-5 in the differentials, per the manual.

Exactly, with no limited slip/posi additives or friction modifiers if you want the G80 locker to operate properly (if the truck is equipped with one). Current GM recommended oil is part number 88900401 (ACDelco 10-4016), 75W-90 synthetic. This stuff:

www.amazon.com/Genuine-88900401-75W-90-Synthetic-Lubricant/dp/B00BK7LXGM

I’ve never seen any evidence that synthetics ever CAUSE leaks.

X2. My '97s have both been running "synthetic blend" 5W-30 for a couple years now and neither have any significant oil leaks.