which oil for a Yanmar YSB

Joined
Jan 28, 2021
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i have a 1977 yanmar YSB8 in my sailboat; I had been using up a small stash of old Delo 30W rated CF that came with the boat, but I haven't been thrilled with starting performance in cold weather. the engine starts great in the summer when it's over 50F or so (and starts very easy in our 95F heatwaves!), but in the winter we get down into the low 30s and it's sluggish to get it going. this is a flat-tappet, raw water cooled engine and it runs cold; operating temperature is set by a 140F thermostat. there is no filter, only a stacked-plate strainer.

I tracked down a copy of the owner's manual for this engine, and it is hopelessly out of date, specifying single-weight oils for the most part:
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my concerns are as follows:

1) need to use an oil available in quarts, ideally off the shelf. storage is tight and its a lot easier to stow quarts.

2) suitability for wet-clutch. the tranmission uses a single disk clutch and the manual calls for using the same oil as in the engine; i'd like to continue that for simplicity's sake. in reading i've been lead to believe that the friction modifiers found in API CG and later specs are not acceptable for the clutch.

3) ease of starting in cold weather. the only straight weight oil i've found locally is the 30W; I'd like to try a 20W l but i can't get it.

delo 400 SDE or rotella T4 15w40 look OK, but i'm not sure it will be much of a difference from the straight 30w in viscosity at starting temp & i'm a little worried about the wet clutch with the CK rating.

if I have to order in oil, would Amsoil's ASF Synthetic Small Engine 10W40 be a decent choice? it looks good in terms of viscosity, it meets CD/CF and it's OK for the wet clutch. but i keep reading on forums that I shouldn't use synthetic in the Yanmar.

or I could stick with the delo 400 30w and just put up with the hard starts; it's only two or three times a year.

i'm getting a bit confused trying to figure out which oil i should try. thoughts?
 
Joined
Jun 11, 2003
Messages
757
Location
Hedgesville, WV
The Rottela T4 and T6 both met the spec for JASO MA and wet clutch performance. If you are worried about pump-ability their is a T6 15w40 full synth that should pump better at low temps than the conventional and a T6 5W40 if you are worried about real cold temps.
 

mcpusc

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Joined
Jan 28, 2021
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OT: what kind of sailboat?
a Yamaha 25II; she's one of the smallest inboard sailboats i've ever seen, and it makes her a great pocket cruiser. i keep her on lake union in Seattle and aspire to cruise the San Juans in a year or two!

The Rottela T4 and T6 both met the spec for JASO MA and wet clutch performance. If you are worried about pump-ability their is a T6 15w40 full synth that should pump better at low temps than the conventional and a T6 5W40 if you are worried about real cold temps.
thanks! i missed that T4 and T6 have the MA rating, those sound like the way to go. i'm not concerned about temps much below 30F so i don't think the 5W-x is needed.

any thoughts on T6 synthetic vs T4 conventional? are the concerns about switching an old engine to synthetic a thing of the past?
 
Joined
Jun 11, 2003
Messages
757
Location
Hedgesville, WV
I would not worry about the switch, its not like going from a conventional to a gp5 oil. The bulk of most synthetics is still gp3 or 3+. I dont think the T6 would be any better than the T4 except it might have a bit better pumpability at low temps and provide better oxidation and sludge control in extreme conditions.
 

mcpusc

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Joined
Jan 28, 2021
Messages
3
I went with the T6 5w40 full synthetic. The engine is noticeably easier to spin by hand now; and the starter is able to spin rather faster than it could before, which makes a big difference to starting - the engine starts first attempt now, even when fully cold soaked.

Thanks for the help!
 
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