Oil type viscosity / Regular or Synthetic ?

Dave R · 07-08-2021, 08:24 AM

Your 4.3 is an automotive-based Chevy without any smog controls like a catalytic converter. If you want the best protection, use diesel oil instead of gasoline powered auto or marine oil. Non-diesel oils are formulated with far less Zinc Dialkyl Dithiophosphates or ZDDP due to the fact that ZDDP damages gasoline engine catalytic converters if it makes it into the combustion process. Since you don't have a catalytic converter, you can get the advantage of a lot of ZDDP (which is fantastic for preventing engine wear), so you might as well do so. The best part of the whole deal is that diesel oil is typically a lot cheaper than gasoline powered auto oil.

As far as viscosity goes, the number preceding the W is irrelevant as along as it is lower than or equal to what the manufacturer recommends. 5W-40 protects at high temperatures just as well as 25W-40. The 40 is the key parameter.

Unless you are putting more than 200 hours on your oil between changes, or you are using the boat in weather well below freezing, there's no advantage to synthetic or synthetic blend oil.

I'm a huge fan of Shell Rotella T4 15W-40 in applications like yours, I buy it in 5 gallon buckets and it's great stuff for very little money. If you really want synthetic, T6 5W-40 is also available.

For those that have marine engines with catalytic converters, you really need to use oil that meets or exceeds the recommendations of the engine builder. Don't use diesel oil.

BroadHopper · 07-08-2021, 03:17 PM

I agree with DAVE R. I use Rotella exclusively in my 454 Mercruiser. 1200+ hours before a rebuild. 15w-40.

bigdog · 07-08-2021, 03:34 PM

I concur with BroadHopper and DaveR, Shell Rotella T4 15W-40, what I've researched is a great and 'under-rated' oil.

8gv · 07-08-2021, 03:50 PM

Quote:

Originally Posted by BroadHopper

I agree with DAVE R. I use Rotella exclusively in my 454 Mercruiser. 1200+ hours before a rebuild. 15w-40.

1200+ hours!!!

You deserve the boat owner's self discipline award.

Most owners move on to the next boat much sooner than that.

LikeLakes · 07-08-2021, 07:43 PM

Quote:

Originally Posted by Dave R

I'm a huge fan of Shell Rotella T4 15W-40 in applications like yours, I buy it in 5 gallon buckets and it's great stuff for very little money. If you really want synthetic, T6 5W-40 is also available.

BTW there is a T6 15W40 now too. I had read about it, then picked up a jug by accident and didn't notice until I went to use it. I run 5W40 T6 in my Duramax.

Dave R · 07-09-2021, 08:56 AM

Quote:

Originally Posted by LikeLakes

BTW there is a T6 15W40 now too. I had read about it, then picked up a jug by accident and didn't notice until I went to use it. I run 5W40 T6 in my Duramax.

I ran 5W40 T6 in my Duramax when I had it, too. Great oil and easy Winter starts. I run T4 in my boat's Caterpillar engines though. They each take 4 gallons, they don't run in the Winter and I don't do extended change intervals, so T4 is the best choice for me. I love that Walmart always has it in stock and they take 5 gallons at a time for recycling.

LikeLakes · 07-09-2021, 09:45 AM

Motor oil is literally the only reason I ever go to Walmart! Agree T6 is a great oil for diesels in New England.

I'm totally on board with those that know more than I do about oil for the Merc 4.3. I answered the OP the way I did because when in doubt, do what the manufacturer recommends is a good plan, with a few exceptions here and there.

bigdog · 07-14-2021, 02:49 PM

Having an issue finding Shell Rotella 20Wx40 T4 at Walmart, they only stock 15W, would this be OK?

Is there that much difference?

NH.Solar · 07-14-2021, 07:20 PM

Really good info from both Dick R and Likelakes. The second number in the designation is a bit more critical than the first as it is the base weight and viscosity when the oil is at running temperature. The first number is the cold viscosity and while it is best to be close it isn't as critical. Due to the cold viscosity additives the 15W will crank ever so slightly better than the 20W but it really affect your performance or longevity at all.
I used to run only Rotella in all of my engines out of habit because it was one of the last blends to have zinc additive but now use Walmart's Supertech brand full synthetic. Zinc is only really important when you have an engine with solid lifter cam and unless you have an old muscle car you needed be concerned about it. I do run full synthetic in everything, including my Duramax, Kubota, and vehicles simply because it cranks so much better when cold and isn't really that much more costly.
I have a mini-excavator that would crank but not start if it was below 30 ...until I changed both the engine and hydraulic oils to full synthetic. Now I can get it to start and move it out of the way even if it is only 20' out. pretty convincing. Second, I used to have a Vmax motorbike that had a common sump for both the engine and tranny and once I switched to full synthetic I found that it shifted much easier when the oil wasn't fully warmed up.

LikeLakes · 07-14-2021, 09:26 PM

Quote:

Originally Posted by NH.Solar

Zinc is only really important when you have an engine with solid cam rollers

You mean flat tappet engine, like a 351 Ford for example. Not meaning to nit-pick.

Dave R · 07-17-2021, 04:03 PM

Quote:

Originally Posted by bigdog

Having an issue finding Shell Rotella 20Wx40 T4 at Walmart, they only stock 15W, would this be OK?

Is there that much difference?

Yup, it's just fine.

07-08-2021, 03:17 PM	#2
BroadHopper Senior Member Join Date: Oct 2004 Location: Laconia NH Posts: 5,616 Thanks: 3,245 Thanked 1,115 Times in 801 Posts	Rotella I agree with DAVE R. I use Rotella exclusively in my 454 Mercruiser. 1200+ hours before a rebuild. 15w-40. __________________ Someday may never be an actual day. Last edited by BroadHopper; 07-08-2021 at 03:18 PM. Reason: spelling

07-14-2021, 07:20 PM	#9
NH.Solar Senior Member Join Date: Jan 2017 Location: Jackson Pond, New Hampton Posts: 247 Thanks: 48 Thanked 142 Times in 79 Posts	Really good info from both Dick R and Likelakes. The second number in the designation is a bit more critical than the first as it is the base weight and viscosity when the oil is at running temperature. The first number is the cold viscosity and while it is best to be close it isn't as critical. Due to the cold viscosity additives the 15W will crank ever so slightly better than the 20W but it really affect your performance or longevity at all. I used to run only Rotella in all of my engines out of habit because it was one of the last blends to have zinc additive but now use Walmart's Supertech brand full synthetic. Zinc is only really important when you have an engine with solid lifter cam and unless you have an old muscle car you needed be concerned about it. I do run full synthetic in everything, including my Duramax, Kubota, and vehicles simply because it cranks so much better when cold and isn't really that much more costly. I have a mini-excavator that would crank but not start if it was below 30 ...until I changed both the engine and hydraulic oils to full synthetic. Now I can get it to start and move it out of the way even if it is only 20' out. pretty convincing. Second, I used to have a Vmax motorbike that had a common sump for both the engine and tranny and once I switched to full synthetic I found that it shifted much easier when the oil wasn't fully warmed up. __________________ Peter NH Solar Last edited by NH.Solar; 07-15-2021 at 05:03 AM.

07-08-2021, 08:24 AM	#1
Dave R Senior Member Join Date: Jul 2004 Posts: 2,985 Thanks: 246 Thanked 744 Times in 444 Posts	Your 4.3 is an automotive-based Chevy without any smog controls like a catalytic converter. If you want the best protection, use diesel oil instead of gasoline powered auto or marine oil. Non-diesel oils are formulated with far less Zinc Dialkyl Dithiophosphates or ZDDP due to the fact that ZDDP damages gasoline engine catalytic converters if it makes it into the combustion process. Since you don't have a catalytic converter, you can get the advantage of a lot of ZDDP (which is fantastic for preventing engine wear), so you might as well do so. The best part of the whole deal is that diesel oil is typically a lot cheaper than gasoline powered auto oil. As far as viscosity goes, the number preceding the W is irrelevant as along as it is lower than or equal to what the manufacturer recommends. 5W-40 protects at high temperatures just as well as 25W-40. The 40 is the key parameter. Unless you are putting more than 200 hours on your oil between changes, or you are using the boat in weather well below freezing, there's no advantage to synthetic or synthetic blend oil. I'm a huge fan of Shell Rotella T4 15W-40 in applications like yours, I buy it in 5 gallon buckets and it's great stuff for very little money. If you really want synthetic, T6 5W-40 is also available. For those that have marine engines with catalytic converters, you really need to use oil that meets or exceeds the recommendations of the engine builder. Don't use diesel oil.

The Following 2 Users Say Thank You to Dave R For This Useful Post:
bigdog (07-08-2021), BroadHopper (07-08-2021)

07-08-2021, 03:34 PM	#3
bigdog Senior Member Join Date: Apr 2006 Location: Central MA-Gilford Posts: 1,453 Thanks: 347 Thanked 124 Times in 98 Posts	I concur with BroadHopper and DaveR, Shell Rotella T4 15W-40, what I've researched is a great and 'under-rated' oil.

07-09-2021, 09:45 AM	#7
LikeLakes Senior Member Join Date: May 2021 Posts: 339 Thanks: 50 Thanked 92 Times in 66 Posts	Motor oil is literally the only reason I ever go to Walmart! Agree T6 is a great oil for diesels in New England. I'm totally on board with those that know more than I do about oil for the Merc 4.3. I answered the OP the way I did because when in doubt, do what the manufacturer recommends is a good plan, with a few exceptions here and there.

The Following User Says Thank You to LikeLakes For This Useful Post:
Dave R (07-10-2021)

07-14-2021, 02:49 PM	#8
bigdog Senior Member Join Date: Apr 2006 Location: Central MA-Gilford Posts: 1,453 Thanks: 347 Thanked 124 Times in 98 Posts	Having an issue finding Shell Rotella 20Wx40 T4 at Walmart, they only stock 15W, would this be OK? Is there that much difference?

The Following User Says Thank You to LikeLakes For This Useful Post:
NH.Solar (07-15-2021)