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Old 22-05-2020, 15:34   #61
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Re: Shell Rotella T6 5W-40 Full Synthetic Diesel for Yanmar 3GM30F

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Originally Posted by gleason99 View Post
i changed to T6 last November and after about 150 hours now my rear oil seal is leaking. Coincidence? The engine has about 4000 hours and I had been using normal non-synthetic Shell Rotella before this. Anybody have any thoughts on this?
Way back 30 or 40 years ago when synthetics first came out, that was always given as a reason to not use synthetic, it will make your engine leak.
I’m sure decades before that when detergent oil first came out, the same
thing was said, and there may be some truth in it, if your not so good at changing oil then it’s likely that you get a coating of sludge in the engine and a good high detergent oil can clean things out, and your engine leaks.

But also in this case the 5W oil is very thin at cold temps and will likely leak where a thicker 15W weight oil may not. Cold as in engine not warm, not meaning actual cold.
By the way Rotella T6 is now available as a 15W-40 oil also, which for most boats is likely to be a more applicable grade.

Synthetic oil will not make your engine leak, and yes an engine will also break in properly on synthetic oil, although many manufacturers still recommend against it for the first 500 hours or so, which goes to show me that manufacturers are not always a reliable source
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Old 22-05-2020, 15:50   #62
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Re: Shell Rotella T6 5W-40 Full Synthetic Diesel for Yanmar 3GM30F

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Originally Posted by Tenedos View Post
Ahoy sailors,

Hopefully a simple question for those who know what this is all about. I am about to change oil for my 3GM30F Yanmar and was looking in what to use. I am not sure what all those numbers mean on oils

Is Rotella T6 5W-40 Full Synthetic Diesel good for it? I read in the forums that full synthetic might be an overkill for old engines but I don't mind an overkill as long as it doesn't hurt the engine.

I would rather go with a robust oil and pay a bit extra than have doubts.

Any recommendations?

P.S. If it matters at all, boat is in San Francisco Bay.
Shouldn't have any problems, per se, with that oil. You'll see a lot of chatter about synthetics, especially some of those automotive versions (such as 0-30 etc) but don't forget that they have to deal with emissions and economy regulations. Lower viscosity means less internal friction and pumping losses. Largely irrelevant for your purposes.
In my case, I changed to 15W-40 but use John Deere Plus 50 brand. It has an "additive Package" (that's oil engineer speak for chemical mixture) that is good for twice the usual change interval. It's a little more expensive but I cruise in remote areas and it gives me a little peace of mind if I have to run past my usual interval.
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Old 22-05-2020, 15:54   #63
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Re: Shell Rotella T6 5W-40 Full Synthetic Diesel for Yanmar 3GM30F

I am pretty sure I used the 15-40 Shell Rotella T6. Actually one of my crew who I told to go get some Shell Rotella brought me The Synthetic because she said that is all they had. So I said why not it should be better, now I am not so sure.
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Old 22-05-2020, 16:52   #64
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Re: Shell Rotella T6 5W-40 Full Synthetic Diesel for Yanmar 3GM30F

I tried to ignore this thread but I couldn't help myself. lol
Oil is like guns & politics but here we go.
Your 3gm30F is likely getting a bit worn so the oil clearances will be opening up. I'll post a study. You dont have to read it all just scroll down to the chart showing oil film thickness with different viscosities. Make of it what you will.
I would run 20w-60 synthetic diesel oil in mine if I wasn't cheap but I use 20w-50 CF mineral. Yanmar says 30 CD but I'm certain I will not have a problem related to using 20w-50 instead of straight 30.
https://www.substech.com/dokuwiki/do...ngine_bearings
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Old 22-05-2020, 17:06   #65
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Re: Shell Rotella T6 5W-40 Full Synthetic Diesel for Yanmar 3GM30F

It doesn't matter IMO. If you put 100 hours per season and change it once a year then T4 is fine. If you put 200 hours a year and only want to change it once a year then T6 synthetic might be a better choice. The motor will rust out before it wears out in most cases.
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Old 22-05-2020, 17:08   #66
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Re: Shell Rotella T6 5W-40 Full Synthetic Diesel for Yanmar 3GM30F

You could cover all bases with T5 synthetic blend.
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Old 25-05-2020, 15:09   #67
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Re: Shell Rotella T6 5W-40 Full Synthetic Diesel for Yanmar 3GM30F

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Originally Posted by Caliber40 View Post
Modern diesels are built for low emissions and increased fuel economy. It's time to move out from under the shade tree and use a modern hi tech lubricant to match this new technology!
The Yanmar 3gm is an old style engine.
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Old 25-05-2020, 15:12   #68
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Re: Shell Rotella T6 5W-40 Full Synthetic Diesel for Yanmar 3GM30F

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Originally Posted by rognvald View Post
The older Yanmars recommend straight 30W oil. I have used Shell Rotella T straight grade oil for over 20 years/2400 hours in my 3GM30F. If you change your engine oil every 100 hours, I would think the difference in using full synthetic vs regular oil might be a moot point. I will continue regular oil changes and straight grade non-synthetic oils. Good luck and safe sailing.
Nailed it!
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Old 25-05-2020, 17:20   #69
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Re: Shell Rotella T6 5W-40 Full Synthetic Diesel for Yanmar 3GM30F

Used to have a Land Rover habit, the old ones that tamed Africa. Had something north of 4 of them that I owned each with over 50,000 miles on the odometer. Changed them over to synthetic and never had a problem. FWIW, Rover designed the engine as a diesel and converted it to gas/petrel by using different pistons and head milling if that was ordered. Converted all my cars to synthetic as well as the boat diesels without a problem.
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Old 25-05-2020, 17:53   #70
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Re: Shell Rotella T6 5W-40 Full Synthetic Diesel for Yanmar 3GM30F

Some of the older oil seals could have accelerated wear when exposed to full synthetic oil.
Any way, 4,000 hours is equivalent to 100,000 miles for the car engine, so oils seals could be worn out.
I'm using Rotella T5 in the same engine, so far no problems
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