Oil Question

[Schurkey]

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

First off, almost all the so-called "synthetic" oil being sold now is NOT synthetic. It's plain ol' ordinary dinosaur oil that the oil companies have played Van Halen at high volume until it submits. Lots of what is sold as "synthetic" in the USA (Group III) cannot be sold as "synthetic" in Europe, where the courts have not allowed the oil companies to mislead their customers.

https://www.twinoils.com/news/a-guide-to-base-oil-groups/

www.motor.com/2022/03/are-all-oil-groups-the-same-oil-differences-101-for-service-and-parts-professionals/

Are All Oil Groups the Same? ‘Oil Differences 101’ for Service and Parts Professionals | MOTOR

When performing oil changes, technicians typically reference and adhere to vehicle manufacturer specifications.

www.motor.com

Thousands of years ago, when oil sold as "synthetic" actually was synthetic/Group IV, (primarily Amsoil, but some other niche companies also) the synthetic oil would dissolve carbon deposits and varnish near the seals and gaskets; when these "false seals" went away, the ancient rubber, cork, and rope seals would start leaking. MAYBE there was some interaction with the actual synthetic oil, and/or the additive package that caused leakage past old seals...but that's beyond my pay grade.

Short story: No real reason to be scared of "synthetic" oil, 'cause most of it is falsely-advertised dinosaur oil that's been refined 'n' processed. The genuine synthetic oil is no enemy of gaskets and seals in good condition. MAYBE genuine synthetic oil leaks faster from damaged/worn-out seals and gaskets, maybe not. The better plan is to fix the crappy seals and gaskets, and use a top-grade oil

 

[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.

Thanks for all the comments. I'll switch to the 5W30 synthetic. But I'm still wondering about the rear differential and using the 75W140. My RPO codes from the vin say G80-Rear Axle - positraction-limited slip, G75-4:10 ratio with a MT1- 4sp auto tranny THM R2(4L80E). I'll attach some photos of the differential in hopes of getting some confirmation of what I have there. Also, I see a leak around the Diff/Axle area. I found a TSB# 99-04-20-001 suggesting 88-99 C/K model testing has shown that synthetic lubricant will attack RTV sealant and to use only non-synthetic. Not sure if I have a gasket or RTV and if that is related to the leak I'm seeing. Looking forward to additional comments and an education
Thanks

 

[Schurkey]

"14 bolt" full-float 10.5" ring gear axle. Nice unit.

If it has G80 still installed, they'd specify the "Grape Juice" gear lube. ACDelco 10-4016. Wildly expensive.
www.amazon.com/dp/B000QGOX1A/?coliid=I2EQWKHVBIDUK1&colid=2VLYZKC3HBBDO&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it

There IS a bulletin about synthetic gear lube attacking the RTV silicone used in original assembly of those axles prior to a specific break-point. Recommendation is to replace the silicone with mechanical gaskets, and then there's no issue with the synthetic gear lube. Note that this bulletin covers the RTV Silicone and the synthetic gear lube used in original assembly, and maybe the GM-recommended replacements. Permatex--and perhaps others--make an RTV Silicone specifically for use with axle lube. And there's a bunch of companies making synthetic gear lube. The bulletin covers ONLY the RTV and the gear lube supplied by GM. Other products might not have poor interaction.

Given your heavy towing loads, I wouldn't be worried about the thicker gear lube so long as you aren't having problems with the G80.

 

[1997]

leak is because "It has 260k/160 miles" and it's 27ys old ........ imo.
nice rig.

 

[someotherguy]

Thanks for all the comments. I'll switch to the 5W30 synthetic. But I'm still wondering about the rear differential and using the 75W140. My RPO codes from the vin say G80-Rear Axle - positraction-limited slip, G75-4:10 ratio with a MT1- 4sp auto tranny THM R2(4L80E). I'll attach some photos of the differential in hopes of getting some confirmation of what I have there. Also, I see a leak around the Diff/Axle area. I found a TSB# 99-04-20-001 suggesting 88-99 C/K model testing has shown that synthetic lubricant will attack RTV sealant and to use only non-synthetic. Not sure if I have a gasket or RTV and if that is related to the leak I'm seeing. Looking forward to additional comments and an education
Thanks

Trust me.. the 75W140 spec for rear axle gear oil refers to the Dana 80 axle which you do not have. In the realm of GMT4xx trucks, it only comes in the 3500HD - which is a beam front axle, 4 wheel disc, 19.5" wheel, heavy duty frame, 15K GVWR chassis cab truck most often seen with a utility bed, wrecker bed, dump bed, etc.

I can understand the confusion from the manual as it's somewhat easy to misinterpret that spec when you're not familiar with that model.

You must be registered for see images attach

Richard

 

[Pinger]

A small point re increasing viscosity from recommended. The full expected gain doesn't materialise because the additional churning with the heavier viscosity and consequent heat tends to thin the oil. There will still be an overall gain in viscosity but not necessarily what was expected.

 

[Islandwhiteknight]

Trust me.. the 75W140 spec for rear axle gear oil refers to the Dana 80 axle which you do not have. In the realm of GMT4xx trucks, it only comes in the 3500HD - which is a beam front axle, 4 wheel disc, 19.5" wheel, heavy duty frame, 15K GVWR chassis cab truck most often seen with a utility bed, wrecker bed, dump bed, etc.

I can understand the confusion from the manual as it's somewhat easy to misinterpret that spec when you're not familiar with that model.

You must be registered for see images attach

Richard

Thanks for the extra insight. So not a Dana. Is mine what's referred to as a locker? If I recall correctly, no additive to be added to the gear oil.

 

[Schurkey]

Is mine what's referred to as a locker? If I recall correctly, no additive to be added to the gear oil.

If it's G80 in a GMT400 vehicle, it's a locker.

Yes, do not add friction modifier to the gear lube.

 

[LVJJJ]

I agree with Schurkey, but wanted to add that Mobil 1 is also a synthetic, so only two oils actually synthetic. Apparently other oil companies sued someone and gained the right to call their oils "synthetic." They are super refined oils that don't do much more than Havoline 30 used to do. I use Mobil 1 for everything except for my 65 Chevy van with a 292 I built in the '80's for which I use Valvoline VR-1 racing oil in with lots of zinc and phosphorus.

 

[CrustyOldDog]

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.

Rotella T6 is not an SAE S (automotive) rated oil. It is only SAE C rated for diesel engines. T4 and T5 the same. If you want the advantages of diesel rated oil in a gasoline engine, it should also be S rated. There are several available.