Thinner oil for raw water cooling?Any

[loco]

probably a well-trodden path, but I’m looking into engine oils again, as it’s time to change my oil.

ive been using motul 15w50 marine oil, but I’m beginning to think it might be a bit thick, as all the other inboard oil I see for sale locally is usually closer to 10w30.
As I’m running a 4.3 v6 in UK waters, and raw water cooled, the engine will be running relatively cool too. Or, is it just better to have a higher oil pressure for marine use. I’ve got old Volvo gauges, so just know the needle is in the green area, rather than a specific pressure.

Any thoughts on what oil to use? I see a lot of quicksilver oil being sold in the marine stores, so was considering going for that this year.

cheers

 

[Scott Danforth]

quit trying to over think it. boat motors are like truck motors going up hill all the time pulling a load

use 15W40 diesel oil.

 

[Chris1956]

Gee, what does the engine manufacturer recommend? Boat engines are usually recommended to run on a FC-W rated oil. MerCruiser usually recommended their Quicksilver 15W-40 oil.

 

[Lightwin 3]

Gee, what does the engine manufacturer recommend? Boat engines are usually recommended to run on a FC-W rated oil. MerCruiser usually recommended their Quicksilver 15W-40 oil.

Second that. So does Volvo.

 

[loco]

Thanks. The manual just states:

SAE 30 (70F and Below); SAE 40 (70F and Above)

So, a whole range of multigrade oils would work, but I'm just curious which. 15w-40 sounds good.

Presumably no major differences in the make-up of diesel oils - additives/zinc content/etc?

Thanks again, all.

 

[Silvertip]

Weight of the oil (it's viscosity) has no bearing on oil pressure except perhaps a bit at idle. The oil pump has a pressure relief valve in it so there is no way you can "over-pressure" the engine as the relieve valve releases at a preset value.

 

[Chris1956]

OK, so no multi-viscosity automotive oils were recommended by the OEM, as I expected.

If you dig a bit, I think you will find an updated recommendation from MerCruiser for their FC-W marine oil 15W-40, which is what I used on my 4.3LX. The thinking is that automotive Multi-viscosity oils do not stand up to the load on a boat engine. FC-W marine oils are expressly formulated to do that.

 

[mr 88]

If you have a thermostat installed,which you should, it wont run any cooler because its raw water cooled vs a closed system.

 

[frantically relaxing]

Weight of the oil (it's viscosity) has no bearing on oil pressure except perhaps a bit at idle. The oil pump has a pressure relief valve in it so there is no way you can "over-pressure" the engine as the relieve valve releases at a preset value.

But you can 'under-pressure' it with oil that's too light, especially in older motors, and especially at idle... light oil can just leak out past worn bearings, leaving not enough oil for the engine parts at the tail end of the oil flow run....

 

[QBhoy]

Mercruiser oil is 25w-40. Not 15w-40. I use the quicksilver (Merc) synthetic blend 25w-40. Good in both Mercruiser and VP engines like that. Seems to hold its self and not degrade as quick as the normal stuff.
I run in the U.K. too...as about as cold as it gets in Loch Lomond waters. Thermostat should take care of any worries about low temps. Engine temps Should be the same as anywhere else really.
There is a guy on the U.K. eBay that sells the synthetic stuff for £35. Great price and improves oil pressure. You don’t want a 50w in the U.K. either.

 

[loco]

I was about to buy the quicksilver diesel 15w40 oil, but reading the back, it specifically said to not use it in petrol engines, so I put it back. I think I’ll go for the quicksilver 25w40 synth blend.

Im running a thermostat, but a cooler one for raw water usage.

cheers!

 

[Chris1956]

The OEM's cooler thermostat in the raw water cooled motors helps keep the rust down, in the manifolds. You are using one of those right?

 

[WIMUSKY]

Thanks. The manual just states:

SAE 30 (70F and Below); SAE 40 (70F and Above)

So, a whole range of multigrade oils would work, but I'm just curious which. 15w-40 sounds good.

Presumably no major differences in the make-up of diesel oils - additives/zinc content/etc?

Thanks again, all.

Actually, diesel oil such as Rotella does have a higher zinc content, but that's a good thing.....

 

[loco]

The OEM's cooler thermostat in the raw water cooled motors helps keep the rust down, in the manifolds. You are using one of those right?

yeap!

 

[QBhoy]

Going back to the U.K. thing...as mentioned, I use the Merc quicksilver 25w-40 synthetic blend. It’s decent stuff, but mainly because it looks good in the service file.
If I had your engine, I’d either use that or more likely the Rock Oil MP4 synthetic oil. Comes in 5l tub and there isn’t better stuff than rock oil over here. Specs are far better than any quicksilver stuff.

 

[QBhoy]

https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/370976333248