Hey everyone, I’ve been reading a lot about this but still can’t make up my mind. I just bought a beautiful 2003 F350 Super Duty Lariat with a 7.3 Powerstroke at 230k miles. The previous owner has kept great records, and it looks like the truck has always used T4. I’m in NJ, and I was planning on switching to T6 with 5w-40 to help her out, along with oil and fuel filter changes. I’m also thinking about using some Seafoam before the oil change.
But I’m a bit concerned. Some people say switching from dino oil to synthetic at 230k miles could be a big change for the engine. It runs really well after sitting for a while, so I don’t want to mess that up. I use the truck mostly for highway driving (100+ miles daily) with occasional heavy towing. It’s mostly stock except for an MBRP exhaust, and there’s no blow-by, leaks, or any diesel smell in the oil. The truck has been really well maintained with oil changes every 5k miles.
Skip the Seafoam, I wouldn’t recommend that. Try an Archoil package instead. I’m not the biggest fan of T6, but you’ll be fine if you switch to synthetic.
Finch said:
Skip the Seafoam, I wouldn’t recommend that. Try an Archoil package instead. I’m not the biggest fan of T6, but you’ll be fine if you switch to synthetic.
I get where you’re coming from. I’ve heard some old-timers say it might be too much detergent in the synthetic oil, and that regular oil has minerals that help with wear. Maybe I’ll try T5 first and see how it goes.
The difference between synthetic and conventional oil is really just the base stock. The additives like zinc, moly, and others depend on the API rating, not the type of oil. Synthetic oil doesn’t magically clean an engine. T6 is a good option and will help with cold starts, and a CK-4 rated 10w30 is another option. With a HEUI system, 5w40 might shear a bit, but it won’t cause any major issues.
Daryn said: @Cody
Good info! What do you mean by shear though?
When a 5w40 oil shears, the viscosity breaks down, and it ends up acting like a 30 weight oil. This happens because of the viscosity improvers in multi-grade oils. The 7.3 engine is pretty tough, so a little shear doesn’t cause much wear as long as you change the oil regularly.
@Cody
Got it! I always thought the first number was for cold starts and the second for operating temperature. I’ve heard about shear before but didn’t understand it fully.
Since 30 weight is fine for these engines, would 5w40 in the winter (with some shear) and 15w40 in the summer be a good idea?
@Daryn
Yes, you’re right. The first number is the cold start viscosity, and the second is the operating temperature viscosity. 15w40 is less likely to shear, but it can be too thick for cold weather in a HEUI system. I’d stick with 5w40 in winter, then switch back to 15w40 in warmer months. I run 15w40 in summer and 10w30 in my 6.7.
Your truck won’t care. I wouldn’t worry about it. I’ve been using Valvoline Premium Blue recently, but honestly, any oil you change regularly will work fine.
I’d recommend Schaeffer’s 15w40 full synthetic over T6, but it’s up to you. Honestly, you’re overthinking this. Your truck isn’t going to care about switching from synthetic blend to full synthetic. The old oil will stay in the heads, fuel injectors, and high-pressure pump, and will mix with the new oil once the engine runs.
The most important thing is regular oil changes. These older engines get blowby, which deposits exhaust particulates into the oil. It’s the particulates that cause wear, not the oil itself. Changing oil every 3-5k miles removes the contaminants and keeps the engine running smoothly. Synthetic oil might cost more, but you’ll still need to change it regularly. If you’re in really cold weather, synthetic is helpful, but if not, conventional oil works fine. In Canada, synthetic can get really expensive—$180 for 5 gallons of T6!
I lived in Fairbanks, AK, so I know about cold weather and HEUI systems. A 5w40 synthetic is a good option for cold weather. But honestly, I think you’re overthinking it. These engines are solid if maintained. If you’re worried about leaks, I’d recommend keeping a Diesel O-rings kit in your toolbox. I’ve had it saved me a few times.
I promise you, the 7.3 doesn’t care what oil you use as long as you change it regularly. Synthetic or conventional, 5w-40 in cold weather and 15w-40 in warmer weather will work just fine. The detergents in synthetic oil will clean it out a little if it’s gunked up, but if you’ve kept good records, it shouldn’t be an issue. Just make sure you’re changing the oil every 3-5k miles.
I’ve got 535k miles on my 02, and I’ve used both synthetic and conventional oils. No issues.