recently bought a 1979 300sd to run biodiesel in. so far, so good. i've heard and read that running b50 or b100 adds considerable lubrication. if i'm driving mostly highway miles, can i stretch my oil changes out to 5k or more? what if i combine b100 with synthetic engine oil?

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1979 300sBd (that's Bio, baby!) - 170,000 miles

5K is not a stretch for me. I drive mostly hiway and change oil and filter an oil at 5K.

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1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer

B100 in the crankcase is a real bad idea................

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

An oil thread! Oh Boy! I use FULL synthethic and go 9-10,000 between changes. The factory spec (at least for my car) is 5,000 for standard dino oil and if you are doing lots of highway miles you can probably stretch it a bit but you should do an oil analysis around 5,000 miles to see how your particular car and brand of oil is doing. These are LOTS of threads here on the topic and about as many different opinions as there are members. The ultimate choice is up to you based on your time/cost tolerance/comfort levels.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold

Any extended change intervals should be determined by oil analysis....soot loading IS an issue to be concerned with.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

I run the SDL with the 603 to 5K with synthetic. It's soot loading is consistently below 1% at the change.

I run the SD with the 617 to 5K and it's soot loading is above 1%. I ran it to 8K on one occasion and it's soot loading was above 2%.

YMMV.

forgive my ignorance but how does one measure the soot loading?

oil anaylsis....its not something you can do at home....

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

On the newer engines you can probably run 10k miles changes provided you use Amsoil, Mobil 1 ect. However with a 617/616 it doesn't matter what kind of oil you use the soot loading will dictate your change. I would get it sampled at 5k miles before I pushed it any further. I wouldn't run anything but a good synthetic much past 5k.

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2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200

I have run every car (gas) and truck (Diesel) I have ever had 10k per oil change and have not only never lost an engine to an oil related problem but I have never had even the slightest oil related problem. If you drive your car long enough to get the oil hot and keep it hot for 30 minutes or so then you can extend your changes to 10k on almost eny engine. I change my oil, oil filter and fuel filters every 10k, and, as I said, have never had a problem.

What ever works for ya- but remember just because you saw a brown cow doesn't mean all cow are brown, as my mom used to say. I just changed my oil at 16,00 miles (Mobil 1) land had a soot level of 2.6- but I've got a bypass filter installed also.

are you using sythetic or dino motor oil?

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1979 300sBd (that's Bio, baby!) - 170,000 miles

uh, where should you have it done? corner service station? dealership? pep boys? the factory in germany? (next time you're in germany, don't miss the tour of the benz factory . . . well worth the time. i remember thinking "ya, like i'm ever gonna buy a benz". i just didn't know what $1,200 could get me!)

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1979 300sBd (that's Bio, baby!) - 170,000 miles

how much soot loading is considered "acceptable"?

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1979 300sBd (that's Bio, baby!) - 170,000 miles

Well, now, you just asked the question of the day. The oil companies will generally allow up to 2%, however, certain Mobil 1 oils, in theory, can go up to 4%.

Do I believe any of that BS. No I do not.

Soot is abrasive. That is what makes the oil on a diesel coal black. The less soot the better. So, personally, I feel real comfortable when the oil analysis comes back with a soot level less than 1%. If this means that I have to do an oil change at 5K rather than 7.5K, then so be it.

Is there something magic about 1%? Certainly not. I have absolutely no data to back up my feelings on the matter.