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CONFUSED! What's the right Yanmar SD-20 gearbox oil for my motor


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Hi all, I'm a bit perplexed - I've got a Yanmar 1gm10c engine with a Yanmar SD20 sail drive unit.

 

The manual for the sail drive states that when paired with a 1gm10c, the SD20 gear oil should be filled with mineral oil 10w30, and when paired with larger 2YM15, 3YM20, 3YM30  engines it should be run on SAE90 ATF type fluid.

 

Theres a sticker on the Saildrive stating it is a 1GM10C SD20 but contradicts the manual and says it should have SAE90 fluid in it.

 

When I check the dipstick it has red ATF type fluid in it, and I haven't noticed any particular issues, other than occasionally mashing gears when changing to quickly which I suspect is more a function of the dog clutch rather than oil.

 

What fluid / oil should actually be in the SD20 when mated with a 1GM10C? Keen to encourage the SD20 to last as long as possible!

Cheers,

 

Jono

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In the case of Gears only, I would stay away from using a motor oil.  Motor Oils don't have the EP strength of a Gear Oil.
I assume when you said SAE90 ATF, you meant to have a / between them, because there is no such thing as an SAE90 ATF.
Motor oil and gear oil use a very different viscosity rating and the SAE90 Gear is about the same viscosity as a 20W40'ish motor oil. A 10W30 is about the same as a SAE75.
The difference between a gear oil and an ATF is a little more complex. While both can do a similar job and in some cases, do, it is still best to use a Gear oil for gear duty and ATF for Auto duty. in the case of the Gearbox using a High Pressure pump to activate pressure plates, then an ATF is best.
ATF's were used in the past because there were no low viscosity gear oils available. It was the introduction of the close tolerance Japanese gearboxes that the SAE75 gear oil was asked for. This stopped the crunching of gears as you changed.

In your case, the SAE 75 would be fine to use. A Dexron 3 would also likely suffice and hence it's use last time by the looks. The SAE90 for the larger engines is for the slightly higher loads. But the high the viscosity of the Oil, the more power is robbed from friction. So they have come down a rating to the SAE75 to get less power loss and the less power has been deemed acceptable in regards to the lower EP strength of the SAE75

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Comprehensive answer thanks. The oil I bought in the end was a SAE 90 gear oil (instead of ATF fluid) which I think said it was a diff oil or something. So summarising your suggesting the SAE 90 with a lower HP motor will possibly rob some power from the setup? Makes sense given that the Sd20 seems to be designed to work with much more powerful engines.

 

How much power is going to be robbed by a difference between Sae 75 and 90? On a 25ft yacht I don’t have heaps to spare! Noticeable you think? I had the boat up on the stand yesterday so had to put something back in. Fortunately the main thing I was worried about, the potential for water ingress, hasn’t happened so I’m much more comfortable with things now.

 

The old oil I took out was red in colour vs the new stuff was more yellow. Red = ATF?

 

Thanks

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I am not sure you would notice any decrease in Power. It think it would likely be a measurable thing re fuel economy, over time. Not immediately apparent. But if you happen to find it harder to get in and out of gear, you may have to drop to a 75weight.

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In the case of Gears only, I would stay away from using a motor oil.  Motor Oils don't have the EP strength of a Gear Oil.

 

My PRM 260 gearbox specifies 10W40 oil.  I would assume the gearbox is developed with that sort of oil in mind.  Certainly makes it easier to carry the same oil for engine and gearbox.

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Absolutely as IT said. I should have looked at the manual. I did not realise it specified motor oil. It thought it had ATF and the poster was wondering about using motor oil.
The Motor oil will likely be for one reason. There must be brass or copper bushes/bearings or something in the box. It is now very hard to get a Gear oil that does not have Sulpur added. Most of the GL series have been discontinued and the only one really available now is a GL5 and that has a high sulphur content. GL1,2and 3 are gone and GL4 is getting hard to find.
75W gear oil is the same viscosity as 10W40 motor oil.
If you want a bit of extra film strength, I recommend you go for the Pennsoil Synthetic motor oil. It has one of the highest pressure strengths (the ability to hold two metal surfaces apart) on the market.

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Another question,is it the oil or does cable etc need adjustment? 

My yanmar was idling a bit fast which didnt help getting in to gear ,once revs lowered it slipped in/out/reverse

Had disconnected throttle as needed to remove exhaust ,took no notice of throttle setting.

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Likely the engine is idling too fast causing the drive not to engage. Get that sorted, then worry about anything else. Although it is never silly not to double check that the cable is selecting properly by moving the correct distance once engaged, If it does not move far enough, the drive dog that engages may not be engaging enough. Remember that the power of the engine is seen directly on this drive dog

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