Did an oil change a couple weeks back, when it was still relatively cool here. O'Really had a sale on the Rotella T-6 full synthetic oil, and a $17 rebate on a 2.5 gallon jug, making it the same price as the T-4 dinosaur oil. (I usually use T4, or Delvac non-synthetic). So I bought it, changed my oil & filter, and almost immediately noticed a decrease in the oil pressure once it was fully warmed up. Pulled the empty jug out of the trash and took a closer look at the label, the viscosity was 5W40. Regular non-synthetic oil for diesels is usually 15W40. I was back at the parts store a few days later, and looked at diesel oils, every Synthetic version (all brands) was 5W40. So I figured "Ok", then later I looked at my owners manual, and it says use ONLY 15W40 viscosity oil.
The weather has been pretty mild until the past week, and didn't think about it much, but April is the month here where we go from 50's to 90's for high temps. Yesterday it was in the low 80's, and my oil pressure at hot idle was down around 15 PSI. So did I screw myself, and now I need to change my oil again? (Or is this a given that synthetic diesel oil is thinner, but won't cause any harm?) At least I haven't towed with it since then... Anyone out there who is using this lighter-weight synthetic in their diesel, with no issues? (I hate seeing the reduced oil pressure, so I will most likely change it out soon either way, but figured I would ask, gonna be pulling weight next weekend).
As always, thanks in advance for any advice or experiences you can share.

I asked the same question on one of the Ford diesel forums about a year ago - seems the 5W40 synthetic is the go-to for most of them and a lot of people responded that they were using it in their older trucks without any issues. BUT - I have a 7.3 engine and oil is everything to these engines - so I shied away from the 5W40 and went with Rotella 15W-40 synthetic. You can probably run it just fine but you might want to go back to the factory recommended weight on the next change.

Not knowing how many miles are on your truck, I might be more concerned going from a standard Rotella 15/40 to a synthetic oil and what affect it would have on the rings and other rotating parts

you should have no issues, the oil is 5 w(winter) 40. the 40 is the summer weight.
I run the same oil, my hot idle oil pressure is 11 lbs. which is within spec for my 5.9.

Joe

If there is a possibility for a leak synthetic will find it....

This is what I run, all I have run in my 2 diesels...

https://www.amsoil.com/p/heavy...40-ado/#pills-home_0

Difference is on the label.

15w-40 lower oil consumption

5w-40 extreme temp

I use 15w-40, change it every 3k-4k, Florida.

If I were in Sheboygan, I'd use 5w-40 winter.

5w-40 oil gets by the rings / seals on startup, it'll use a little more oil.

This message has been edited. Last edited by: Mike Rietow,

When I tried synthetic in my 5.9 cummins it started leaking at the rear main.

I use synthetic oil in everything. It's not uncommon to see a slight drop in pressure when you switch to synthetic but I've noticed that the oil pressure seems to stay more stable as the engine warms up - with conventional oil, you always have really high oil pressure when you first start up cold but then it drops as the engine warms and the oil thins out. I feel like you don't usually see as much of a hot-to-cold pressure swing with synthetic. I also feel like synthetic seems to come out looking like the same viscosity it went in versus conventional oil that seems to look heavy going in and thinner when you drain. I don't believe it will cause leaks but as other have said if you have a slight leak or weak seal, synthetic will find it! Not an engineer, just my observations after 25+ years of using synthetics in everything from my lawn mower to my racecar AND running it a lot longer than most people are comfortable (70-80 runs in a 584" BBC on alcohol going 7.20's).

Thanks for the feedback, Guys.

Mike, I didn't know they made T6 / Synthetic Rotella in 15W40, all I can find here is 5W40. Thanks for the pics.

Billy, I am also a fan of synthetic oil, that is all I use in my race engines. My previous trucks were gas engines, I used it in them as well. So what viscosity do you put in your tow vehicles during Summer (hot) weather?

Delvac MX

https://www.mobil.com/en-au/co...obil-delvac-mx-15w40

vs

Delvac 1300

https://www.mobil.com/en/lubri...c-1300-super-15w-40/

Delvac has been a staple in the trucking industry since inception.

The 1300.
Low ash formulation meeting API CK-4 and CJ-4 requirements.
Long emissions Aftertreatment life (DPF, DOC and SCR)

It's the only oil i have ever used for the Class 8 industry.. Use it everything i own except for the long car. then it's RP XPR 20W50.

This message has been edited. Last edited by: TD3550,

All I’ve ever run in my ‘15 Duramax is Shell’s Rotella 15w-50 with AC Delco’s PF2232 oil filter. Got 250,000+ on the odometer and still going strong.

quote:

Originally posted by 1320racer:
All I’ve ever run in my ‘15 Duramax is Shell’s Rotella 15w-50 with AC Delco’s PF2232 oil filter. Got 250,000+ on the odometer and still going strong.

Ed, do you mean 15W40? Haven't seen a Rotella 15W50 around here. That is generally all I use, Rotella T4, or the Delvac 1300 that TD3550 mentioned. Always a PF2232 filter. I really like Wix filters, superb quality, use them exclusively on the race cars and muscle cars, but their Duramax filter is undersized, compared to the AC Delco version.

Yes, 15W-40

quote:

Originally posted by 1320racer:
Yes, 15W-40

Synthetic or non synthetic?

quote:

Originally posted by 1320racer:
Yes, 15W-40

You lied.... you said 15w50 before. Which is it?

LOL

In all seriousness, I am switching mine over to the Costco "Kirkland Diesel 15w40" from Delvac at my next oil change. Probably changing all my vehicles over to the Kirkland.

Non

quote:

Originally posted by 1320racer:
Non

That makes me feel better, that is all I have ever put in my DMAX

quote:

Originally posted by head gamez:
I am switching mine over to the Costco "Kirkland Diesel 15w40" from Delvac at my next oil change. Probably changing all my vehicles over to the Kirkland.

Mikey, do you have solid info that the Costco/Kirkland house brand diesel oil is actually Mobil Delvac?
I am a fan of Costco. Smoked a really nice 16 Lb brisket from them yesterday. They sell only USDA PRIME grade, $2.99/LB. The local Kroger store only sells Choice grade, they wanted a dollar more per pound, ($3.99), for a lesser grade of meat. Difference of $16 on 1 piece of meat, and higher quality at that.
So if they can save us a few bucks on an oil change that would be great. I don't think they sell filters though...