MerCruiser 7.4L Flat Tappet

Joined
Jul 8, 2022
Messages
8
We recently purchased a 1996 Cobalt 232 with a 7.4L Merc. We houseboat at Lake Powell annually and wanted a big comfortable family cruiser to add to the mix. The boat is a time capsule and the maintenance records are extensive. She looks like she came off the factory floor yesterday.

As the title suggests, the engine is not a roller. I have looked through the forum and heard what sound to be valid arguments for and against the three main options for lube. Cost is not an issue on a boat which will have an oil change once a year, well within the 100 hour interval.

1) 25w40 Merc/Quicksilver
Pro-It’s recommended by the factory, it’s what has been used and it meets all the marine ratings.
Con-It is the factory fill for boats with catalytic systems, therefore does not have sufficient protection for a flat tappet engine. (I have not seen a recent VOA to either confirm or deny this claim)

2) SAE 40w
Pro- It is also recommended on the owners manual, the boat will only be ran in warm weather and the straight weight oil is more shear resistant.
Con- I have read that the HDEO Detroit rated oils would not be as well suited to the application due to low (zinc/phos). “Old tech, the 15w-40 will provide better protection.” No SAE 40w I can find has the marine specificatios.

3)15W40 HDEO
Pro-Many are available with sufficient zinc/phos (plenty of literature and voa’s to support this) , provides better protection on start up, is what Crusader Marin recommends (some of the longest lived engines on the water.
Con-multi weight oils are specifically not recommended in the manual.

What would you choose and why?

Thanks,
Garrison
 

Attachments

  • AE0B4387-6621-48F7-9F9C-2D7D6C1B8F94.jpeg
    AE0B4387-6621-48F7-9F9C-2D7D6C1B8F94.jpeg
    124.1 KB · Views: 76
  • 2B357CF4-E8E0-4E30-8FDB-1A268B5CEF27.jpeg
    2B357CF4-E8E0-4E30-8FDB-1A268B5CEF27.jpeg
    123 KB · Views: 76
Just because it's FT doesn't mean it needs wild amounts of ZDDP. FT cams that don't have crazy lobe profiles and heavy springs survive just fine on even your basic API level of the stuff. The higher levels are important for break-in, but this has been long broken in.

The 25W-40 may have little to no VII in it too, because that's not much of a Winter rating. If it uses quality base stocks, it's quite possible for it to be a monograde or close to it.

Of course the manual was written back when VII's were much lower quality than they are now, as well as oils in general. I'd personally probably run a 5w-40 in it, but the 15w-40 will be just fine. I like using HDEO's in marine engines, they are OE for many of them and there are plenty of sliding follower HDEO applications still in the HD diesel scene.
 
Just because it's FT doesn't mean it needs wild amounts of ZDDP. FT cams that don't have crazy lobe profiles and heavy springs survive just fine on even your basic API level of the stuff. The higher levels are important for break-in, but this has been long broken in.

The 25W-40 may have little to no VII in it too, because that's not much of a Winter rating. If it uses quality base stocks, it's quite possible for it to be a monograde or close to it.

Of course the manual was written back when VII's were much lower quality than they are now, as well as oils in general. I'd personally probably run a 5w-40 in it, but the 15w-40 will be just fine. I like using HDEO's in marine engines, they are OE for many of them and there are plenty of sliding follower HDEO applications still in the HD diesel scene.
I have put many thousands of hours hauling wake-boarders around in a Malibu and tuna fishing in my saltwater boat with HDEO’s. Just rolled 300k on my 7.3 Excursion. History makes it hard for me to use anything else irrespective of price, but none have been 26 years old.
 
I have put many thousands of hours hauling wake-boarders around in a Malibu and tuna fishing in my saltwater boat with HDEO’s. Just rolled 300k on my 7.3 Excursion. History makes it hard for me to use anything else irrespective of price, but none have been 26 years old.
Our Supra is similar vintage, I run Delvac 1 5w-40 in it. I also used it in our Glastron, which was a 70's boat, flat tappet. I agree with your history and wouldn't pay any mind to the fact the boat is 26.
 
We recently purchased a 1996 Cobalt 232 with a 7.4L Merc. We houseboat at Lake Powell annually and wanted a big comfortable family cruiser to add to the mix. The boat is a time capsule and the maintenance records are extensive. She looks like she came off the factory floor yesterday.

As the title suggests, the engine is not a roller. I have looked through the forum and heard what sound to be valid arguments for and against the three main options for lube. Cost is not an issue on a boat which will have an oil change once a year, well within the 100 hour interval.

1) 25w40 Merc/Quicksilver
Pro-It’s recommended by the factory, it’s what has been used and it meets all the marine ratings.
Con-It is the factory fill for boats with catalytic systems, therefore does not have sufficient protection for a flat tappet engine. (I have not seen a recent VOA to either confirm or deny this claim)

2) SAE 40w
Pro- It is also recommended on the owners manual, the boat will only be ran in warm weather and the straight weight oil is more shear resistant.
Con- I have read that the HDEO Detroit rated oils would not be as well suited to the application due to low (zinc/phos). “Old tech, the 15w-40 will provide better protection.” No SAE 40w I can find has the marine specificatios.

3)15W40 HDEO
Pro-Many are available with sufficient zinc/phos (plenty of literature and voa’s to support this) , provides better protection on start up, is what Crusader Marin recommends (some of the longest lived engines on the water.
Con-multi weight oils are specifically not recommended in the manual.

What would you choose and why?

Thanks,
Garrison
The manual was written in 1995 or so for a 96. All oils have changed in leaps and bounds since then. I have run Mobil 1 Delvac 5w-40 for 20 years in my Chevy powered boats with zero side effects. The Merc/Quicksiver is a synthetic blend to which I see no point.
 
I do wish the specs were available, or a VOA was posted for some of the new catalytic compliant 25w-40’s. Think I will send in a sample at the end of the season on the current fill. If I go with a 15w-40 HDEO (most likely), think I will make sure it is on Ford’s approved list.
 
Thats a good looking boat.

I would tend to stick with whats been working - in this case the QuickSilver.

If deviating - Agree with Overkill - Quality HDEO 15/40.
Thanks, looking better all the time. Been steadily checking upgrades off the list. Sorted out the ground tackle situation today, wired a Blue Sea battery charging relay and switch so the stereo doesn’t kill the starting battery, some new led courtesy lights, installed a small bottom machine so I can keep the stainless props off the bottom and a new steering wheel today.

Next up, extended swim platform, some teak look woven vinyl flooring and a nice little wine and cheese table for the wife. Should be ready for a cocktail cruise in short order!
 

Attachments

  • 2EF0947B-637D-48C0-99B7-7F266F43C0A3.jpeg
    2EF0947B-637D-48C0-99B7-7F266F43C0A3.jpeg
    97 KB · Views: 34
  • 25C5D432-C83D-4870-88E5-99F27D161C99.jpeg
    25C5D432-C83D-4870-88E5-99F27D161C99.jpeg
    84.5 KB · Views: 34
  • 3F646522-33F5-4ED6-812E-F7F6EC583C05.jpeg
    3F646522-33F5-4ED6-812E-F7F6EC583C05.jpeg
    104 KB · Views: 34
It's oil. Corrosion resistance can't be too bad can it?
It is my limited understanding in marine lube the “corrosion resistance” umbrella encompasses alkalinity and oxidation in a machine that will be in storage more than used, as well as those subjected to a harsh environment. Although much more of a concern for our saltwater boat than this one, it is still a concern. I could be completely wrong, but I don’t believe VR1 is developed with this in mind. More of a run it hard, short drain interval oil? It does look like it checks all the boxes in every other way.
 
Last edited:
I do wish the specs were available, or a VOA was posted for some of the new catalytic compliant 25w-40’s. Think I will send in a sample at the end of the season on the current fill. If I go with a 15w-40 HDEO (most likely), think I will make sure it is on Ford’s approved list.
I do wish the specs were available, or a VOA was posted for some of the new catalytic compliant 25w-40’s. Think I will send in a sample at the end of the season on the current fill. If I go with a 15w-40 HDEO (most likely), think I will make sure it is on Ford’s approved list.
Your Mercruiser engine is a GM engine, why would you care about Ford?
 
Rygrego,
Because, some oils meeting CK-4 standards have been found to lead to accelerated wear in their vehicles and the “WSS-M2C171-F1” spec sets a minimum threshold for ZDDP and phosphorus. Their position letter I believe also stated the CK-4 oils that are not approved were not recommended in older vehicles as well. To answer your question:

1) My tow vehicle is a Ford, and it would be nice to stock one oil for both, (truth be told there is a few other things that get 15w-40 around here too).

2) Ford’s ZDDP/phosphorus threshold is more in line with what is widely recommended for flat-tappet engines and still compatible with my Chevrolet. Which was a concern when I wrongly believe I owned one.😉

In short, there seems to be no downside with using Ford’s spec, and “potentially” something wrong if I don’t. The products listed are the same price or cheaper, so the question really is. Why wouldn’t I use something with that meets WSS-M2C171-F1 spec or has been approved, if using an HDEO?

Garrison
 
Last edited:
I run Amsoil Z-Rod in my flat tappet engines. Changed every other year.

Has high ZDDP, and is readily available. Jist wish they sold it in jugs instead of qts.
 
Back
Top