New engine...when to switch over to synthetic oil?

[coolwind57]

Last year I had an engine built.  I bought a freshly bored (.040") 1993 block and had the machine shop go ahead and fit as a short block with upgraded pistons and all block components.  I had them install a Comp "RV" camshaft and had extensive cylinder head milling to the 1994 HO 7120 I had bought, installing modern Hemi springs in order to match the camshaft lift specs.  

 

Anyhow, I've got a little over 3,000 miles on the engine presently.  I broke the engine in with 30wt break-in oil.  I changed to 10W30 Rotella T4 and I added some ZDTP additive, which I think I ran for 500 miles.  Replaced the oil with another round of Rotella T4 10w30 and again added some ZDTP and have run this to present day, 3K miles.  By the way, I have no worries about zinc and catalytic converters....I don't have one.  

 

The end goal was to at some point go to T6 Rotella synthetic and reap the benefits of the good zinc content for our 4.0s with the known sweet tooth for zinc.  

 

For those of believing in and using Rotella products:   I'm curious of when you guys would do the switchover to T6 (and maybe even would you).  Should I hold off for a few thousand more miles?  Should I transition to T5 synthetic blend and then later go to T6?  At what mileage would you go to T6?  

 

 

[AZJeff]

My machine shop told me to run 10w30 Dino oil for the first 500 miles, then change it.   Then another 1500 miles with the same oil, and change again.  At that point, run what you want.

 

I personally will be running Mobil One 10w30, as the Oil Geek on YouTube says the ZDDP levels on the current blend of that is quite adequate for flat tappet camshafts.

[ghetdjc320]

Rotella is some good stuff. JASO rated as well. I’d stick with it. Don’t see any down side to running the synthetic as it still has zddp.

[coolwind57]

On 9/19/2025 at 5:24 AM, ghetdjc320 said:

Rotella is some good stuff. JASO rated as well. I’d stick with it. Don’t see any down side to running the synthetic as it still has zddp.

Was definitely sticking with Rotella for sure.  Rotella T6 is suppose to have plenty of ZDDP.  I'll probably go ahead and go to T6 here in a couple hundred more miles.  Hopefully, my engine won't develop leaks all over.  The old guys used to say that switching over to synthetic from dino causes leaks, but I'm not so sure if that's really a thing nowadays. 

 

Thanks for the input, fellas.  

On 9/18/2025 at 7:09 PM, AZJeff said:

My machine shop told me to run 10w30 Dino oil for the first 500 miles, then change it.

I was reading to use straight SAE 30 "Break-in" oil.  I used Lucas brand that I ordered from Amazon.  I seem to recall that I started and ran the engine for like 20-30 min or so and I think I then dumped the SAE30.  I filled with Rotella T4 with a bottle of ZDDP additive and ran for 500 miles.

 

I could be wrong, but I think that's how I did it.  Regardless of whether or not I did it this way, I did ask the machine shop guy for his concurrence and he agreed with what I had read and planned. 

 

Different ways to skin a cat.   Thanks for the input.  

[Eagle_SX4]

12 minutes ago, coolwind57 said:

The old guys used to say that switching over to synthetic from dino causes leaks, but I'm not so sure if that's really a thing nowadays. 

Take this with a grain a salt as I am no expert. From what I have read and understand, synthetic oils have more detergents in them which helps clean the engine and can remove old tarnished oil that was preventing you from seeing a failed seal. It won't cause new leaks but might cause small leaks to become bigger. 

[coolwind57]

20 minutes ago, Eagle_SX4 said:

Take this with a grain a salt as I am no expert. From what I have read and understand, synthetic oils have more detergents in them which helps clean the engine and can remove old tarnished oil that was preventing you from seeing a failed seal. It won't cause new leaks but might cause small leaks to become bigger. 

Makes sense.  

 

By the way, did your 87 come with the Dana 44 or did you put it in?

 

[Eagle_SX4]

1 minute ago, coolwind57 said:

 

By the way, did your 87 come with the Dana 44 or did you put it in?

It came with the truck. I am fairly certain it is the factory axle it had 3.07 gears in it with the tag still on the diff cover when I bought it.