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Full synthetic oil

I have a jeep with a 4.0l I am thinking about switching to full synthetic oil. I run a blend now. Would it be bad to switch to a full synthetic oil?

What does the owners manual call for? If it does not call for synthetic no need for it, you can use synthetic, it will just cost more. Not bad for your Jeep, but you will not get any major benefit.

No it wouldn’t be an issue… But why are you doing this? You adding a Turbo? Or just want the longer change interval ? Well there are several benefits…I was just curious.

If you do switch over…there is nothing you need to do…except…pay attention to the Oil Level…check it often…because you will probably be going many more miles between oil changes than you are used to with conventional oil.

Sounds funny, but people seem to need to be told this obvious bit of info… Better safe than sorry I guess.


I don’t know why anyone would use a synthetic blend oil.

Nowhere on the container does it state what the blend is. Is it 50/50? 75/25? 90/10?

But it really doesn’t matter. A synthetic blend oil is only as good as it’s lesser component. Which is the regular oil.

Full synthetic oil? I think of it as being application specific.

If the engine is turbo charged, if the vehicle does a lot of heavy hauling/towing, if the vehicle is exposed to extreme temperatures, or if the vehicle is raced, there’s a benefit to synthetic oil.

As far as extended oil changes?

You have to remember that the makers of these synthetic oils make these claims. But if you look in any owners manual, nowhere does the manufacturer recommend extending oil change intervals when switching to synthetic oil.


Switching to a full synth won’t hurt anything, but it probably won’t help much either. Your 4.0 engine is an ancient design compared to modern standards and was engineered to run just fine on conventional oils.

Synthetic oils offer benefits like flowing well under extreme cold conditions, and resisting sludge and breakdown under high heat and high rpm. Think a cold Minnesota winter morning or towing a trailer through the Mojave desert in August. Unless you have any of these conditions, conventional oil should suit you just fine.

We get emails for $10 oil changes from dealer on our 2105 civic. Fine print says some models extra. Is that sneaky advertising for non-syn oil changes? No dealer I know offers syn oil changes for $10.

I’m sure that the extra money you will be spending for synthetic oil could be put to better use.

Agree, no benefit, but no harm either. Your choice.

No problem switching but do not increase the oil change interval just because you have more expensive oil in your crankcase.

As pointed out, full synthetic oils are ideal for EXTREME conditions; heat, cold, heavy loads (like in towing), but unless called for in your owners manual, they are not necessary.

Toyo requires synthetic on all the newer cars. I use it in my 99 Camry because the V6 has a known sludging issue. It does not hurt to use it, but it won’t help either. If you extend out your oil change to 10k miles and buy the larger filter for extended intervals than you save time and time is money. Also, it means less disposal and better for the environment. Everybody has opinions on this. So it’s up to you.

As an experiment, I put 10W-30 synthetic in a lawn mower engine that called for straight 30 weight. The lawnmower burned a lot of oil and the full synthetic cut the oil consumption by half and I don’t have nearly as much blue smoke as I had with the straight 30 weight. I would put a new short block on the engine, because the mower has a cast aluminum deck which is in great shape. Unfortunately, parts are no longer available for the mower–not even blades. The full synthetic at my Rural King store is under its housebrand RK label, costs $2.69 a quart–only $1 more than 30 weight dino oil. I would go through 3 quarts of dino a mowing season as opposed to 2 quarts of synthetic oil, so I am not losing any money. However, if the engine didn’t use oil, I wouldn’t spend the extra dollar on synthetic. Keep your oil changed and your Jeep will be fine on dino oil.

I disagree somewhat. I had a Chevy Tahoe with a 350. Winter starts–never extreme, never below 0 Farenheit. With synthetic of the same viscosity rating (5W30) it was easy to tell that the engine got lubrication MUCH faster than with dino. You can’t tell me that’s not better for the engine.

I’d agree, but I think most here would say that starts with the engine below 32F would be helped with synthetic.

Full synthetic if you frequently tow or haul heavy loads.

Full synthetic if you live in a cold climate with nighttime temperatures significantly below freezing.

Otherwise regular oil is fine.

Full synthetic holds up better under high heat and high stress conditions. It also flows better at extremely low temperatures.

Do not extend your oil change interval beyond the manufacturer recommendations if there is still any warranty time on the Jeep. Out of warranty you can decide if you want to push the change interval, but I’d not recommend it.

My local Rural King store sells full synthetic oil under its RK house brand label for $2.69 a quart. It is available in 0W-20, 5W-20, 5W-30, and 10W-30 viscosities. The 0W-20 meets the specifications for my 2011 Toyota Sienna. My guess is that in a couple of years, synthetic oil under major brand names will cost no more than dino oil. If I were still doing my own oil changes, I would use the RK synthetic. As dealers stock more synthetic in bulk, the dino oil will be phased out and a synthetic oil change will cost no more than a dino oil change.

I’ve NEVER used synthetic in any of my wife’s vehicles. Her Accords went well past 300k miles and still running fine when we sold them. I use synthetic on my trucks because I tow and during the winter we take ski trips where it’s not unusual to see -30F.

The old 4.0 is simple and tough. It should be fine with conventional. Most of these will rust away before the engine ever has issues. Rust has been an issue on many I have seen.

That being said, I run synthetic in EVERYTHING I own. I always buy it on sale in large quantities so rarely pay more than a normal conventional change. Why not use it if you can get it for the same price?

My car engines seem to all run smoother on full synthetic and my newest car requires it in the manual. As for my mowers, I have two riders now running a Kohler Command 16 hp engine. Both are nearly identical engines. Both were used when I got them (15+ years old) and used some oil and puffed a small amount of smoke when started. Then I went with the Rotella T6 5W40 synthetic in these. They now don’t use any oil that I can see on the dipstick and there is no more smoke on startup.

I run both these engines HARD and I mean HARD! I often mow heavy grass and have an override on the governor to where I pull a wire to open the throttle plate 100% for maximum power under load. The engines don’t skip a beat and the oil doesn’t even look that dirty or broken down when I change it. I am a believer in this oil for these engines. There is less clatter in the hydraulic lifters too.

Incidentally, Briggs and Stratton now recommends/allows 5w30 synthetic year round in all of their small engines instead of the 30 wt in summer and multi in winter. I’ve changed all of half a dozen small engines to Mobil 1 now due to the simplicity of using a single oil in everything. I’ve noticed no change in oil consumption or anything but no way would I go the 50 hours between changes that they suggest.

I agree with the others though, there is no benefit in switching from dino to full synthetic unless the manual calls for it or the weight demands it, like 0-20. I quit having my oil changed at the Acura dealer and do it myself because they put in a synthetic blend instead of full synthetic. I see no benefit of a blend and a lot of issues due to no standard specifications for it. A couple ounces of synthetic in a quart of dino, and its a blend? Is that OK?

Do not extend the oil change interval unless the oil has an enhanced additive package. You don’t want the oil to turn acidic and damage the engine.

Honestly I like full synthetic oils better then Dino oil. Down fall is you can’t switch back once using full synthetic oil. But I live in montana and we get down to -40°F hereally during winter. And I visit SD a lot so 120°F. Plus I don’t mind paying the extra money for my truck. So it’s really choice preference if your not doing what everyone is telling you.