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Discussion Starter · #1 ·
My question was does anyone use synthetic 15W40 in their DMAX 7800? Can the new Topkicks and Kodiaks with the DMAX 7800 use synthetics? My WT5500 has a DMAX 7800 & the owners manual tells me "Do Not Use Synthetic Oils". This engine is probably a little different than the Dmax 7800s going into the new topkicks (6500-8500) since my truck was completely made in Japan (except for the van body). I do my own maintenance and this question had always bugged me. I like the quicker cold starts with synthetics and extended drain intervals with synthetics. What design makes this engine uncompatible with synthetics?

heres some more history on my truck and some pics:
Bought this 02 GMC WT5500 new in May of 03. Was originally interested in the Topkick but sales pointed me toward this truck. Overall, I am very happy with this truck. I like the larger cab and cabover design (great for tight city streets). The cab is exactly the same as my other larger GMC T8500 tandem truck except for the seats. The only problem I've had was a starter relay failing causing the truck not to start but I was able to get the truck running again by manually wiring 12V direct to the starter (did this for 2 weeks until I got it fixed).

It has a DURAMAX 7800 7.8L IL6, 200HP @ 2400 RPM, 441 lb/ft @ 1500 RPM

The trans. is an ISUZU 6 Spd. OD manual (MLD6Q)
Truck is extremely fast, even though the max spd. on the speedo was 85, it can and usually does go past it (around 88-90 MAX) with minimal pedal effort. This is the first Medium Duty that I feel has enough power!
 

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What kind of fuel mileage do you get with that bad boy?
 

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Discussion Starter · #3 ·
the truck has dual 42 gallon tanks and I'm guessing it to be around the 14-15 (could be more or less...), I haven't really calculated fuel mileage since it mostly runs in NYC on local deliveries with the constant stop and go, etc. Every 2 weeks it has a load going to Philadelphia where I usually fill it up on my way back in New Jersey. I hardly ever fill up the tank in NY, I usually only get $30-40 worth of fuel since its $3 or close to it in all of the stations here. I usually average 300 miles a week. It is an economical truck to own in my opinion. Where as my other truck the T8500 tandem gets around 7 MPG mostly highway runs from NY to Boston or from NY to VA and all points in between.
 

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For some reason the Japanese have been very slow to embrace synthetics. I've been told it has nothing to do with the oil itself, but rather their Engineers are afraid that if they do endorse the synthetics, the truck operators would not do the other maintenence that the the trucks need every 7,500 miles, like grease, checking brakes, belts, hoses, etc. They believe that most operators (especially the Americans) would just run the trucks until 15,000 or so and check everything when they change the oil.

I can't say I blame them, the Engineers are probably right. We'd run the darn things until some ungodly mileage before touching the trucks (because the synthetics would let us) and when something broke because we did maintain it properly, we scream warranty! Been there seen it happen many times.

The synthetic oils won't hurt anything as long as you don't negelect everything else. The differentials already run synthetics and there is a option for synthetics in the Allison transmission as well on the US build products.
 

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Discussion Starter · #5 ·
Maintenace is probably the most important task next to operating the trucks efficiently! After throwing in 38K for the WT5500 and 80K for my T8500 you can believe that I'm gonna stay on top of maintenance! Right now, I use mostly CASTROL Tection Extra 15W40 or standard Rotella T for the WT5500 and AMSOIL 15W40 synthetic diesel oil for my T8500. I seem to really like most of the extended drain interval products that are available nowadays. Just at the last PM, I drained and completely flushed the cooling system of the "green" antifreeze and refilled the truck with ROTELLA ELC coolant. I also changed the manual and rear end lubes with AMSOIL 80W90 gear lube. For some reason, the rear axle lube was all bubbled and foamy. What can cause the gear lube to go all white bubbly and foamy? Did air and water get in?
 

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It sounds like water to me. You might want to check the diff. vent and make sure everything looks OK. Some of them, (I don't remember the T/F-series specifically) have a small cap on top of the vent that could come off if something hit it and then it would allow water intrusion.
 

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just a thought on the synthetic question ...

I own a 02 Harley Super Glide and there is a very big warning about not using Synthetics in my enginge or tranny etc. there have been many documented cases where owners have ignored this warning and as a result had damage due to lubrication problems. Harley only just reently came out with a synthetic that is approved to use in the newer bikes.

Could a similiar reason be behind the engineers not wanting synthetic used in that 7800 motor? just a thought to look into before someone makes an expensive mistake.
 

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Discussion Starter · #8 ·
I agree FaithTrans, I'm just going to stick with regular oil and regular lube services at 3 Mo./ 6K. My truck seems to fall right in the 6k in 3 months interval anyway. I'm not going to use anything that the manufacturer specifically says "NO" to because if something does break, theres going to be no one to help me. I've just been curious as to why the DMAX 7800 is incompatible with synthetics.
 
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