Liquid Molly vs Amsoil vs Kendall

Dec 7, 2010
Corona, CA
When I first purchased my new to me boat I had it serviced and the mechanic put Kendall 20/50 in it and it ran great in regards to the engine temp. Last season I had a different mechanic service the boat and he put a semi-synthetic blend 10/40 made by the manufacturer in the boat and it seemed like last year after a hard run the engine temp would go a little higher and take a little longer to come down. This year I had the mechanic who did the Kendall service the boat again but he used Liquid Molly 20/50 in the boat and so far this year it seems like it got to temp much faster than I'd like given the water temp and air temp. I'm not familiar with Liquid Molly; however, I'm kind of an Amsoil fan boy to be honest. What's the consensus with the liquid molly, should I pull it and put the Amsoil 20/50 in? Boat has a Merc 496HO and we normally boat in Havasu where the ambient temp is 120* and the water temperature is in the 90's so after running the boat at about 4,000 RPM for a while, I want to make sure I have something really good in there.
That engine calls for 25W40 Quicksilver oil. If you don’t want to buy Mercury’s Quicksilver oil then, the other options are Mobil 1 15W50 or a 20W50.
I agree that the oil temperature and pressure should be monitored along with the coolant temperature.
That engine calls for 25W40 Quicksilver oil. If you don’t want to buy Mercury’s Quicksilver oil then, the other options are Mobil 1 15W50 or a 20W50.
I agree that the oil temperature and pressure should be monitored along with the coolant temperature.
I don’t recall the oil pressure changing much, seems like it’s usually around 40 off of memory. I don’t have an oil temp gauge so I really have no idea. You are correct on the 25/40 manufacturers recommendation but I think the extreme heat of Havasu warrants the 50wt.
Of the three I would go with Amsoil. Now if you had Rotella or Valvoline or Mobil then I would say take your pick, all very good oils. As is Quicksilver.

I would not correlate the engine oil with engine temp. Engine temp can be effected by outdoor temp, water temp, how hard you are pushing the engine. And to the thermostat if it was changed.
I only recommend full synthetic.

If the oil PSI is good with a 40 or a 50, then you can use whatever you want in those common grades.

I would try Mobil1 15w50 or their synth Vtwin 20w50, RoyalPurple XPR/HPS 20w50, Amsoil, MPT, Torco, HPL, RedLine equivalents...

LiquiMoly has a bunch of products. Need to know which PN or marketing specific oil that was used. They make 4-6 20w50's. I would have no problem using LiquiMoly oils.

Engine temp? Go thru the cooling system... thermostat, crossovers, exchangers, hoses, pump.....

With those temps, I'd even consider a SAE40 or SAE50 full synthetic oil.
Amsoil 25w40 or Quicksilver 25w40 blend would be my vote too. If you’re concerned about temps, address the actual cooling components: impeller and thermostats.
Merc/Quicksilver also sell a 25/50 syn blend that was formulated for the Verado supercharged outboard and race engines, in your very hot ambient climate that's what I'd use, in fact I started using it in my old Chevy 4.3. It is very expensive but is probably one of the best OEM oils for a high performance marine engine. Alternatively you could use Valvoline VR-1 20/50.

keep in mind that the Mercruiser and Quicksilver products are identical, the difference in labeling is that Merc dealers have to sell the Merc labeled product whereas the Quicksilver product is most often sold elsewhere, including Amazon, I also found it a West Marine.
How old in the impeller? The temps getting progressively hotter season to season would have me suspecting its health, not the oil.
Impellers are a wear item, depending on water conditions, if you boat in shallow water that is sandy, they will need more frequent replacement. I usually run mine for 2-3 seasons then change because you don't want it coming apart and clogging the cooling system.
The other thing to check is the thermostat, make sure it opens at the specified temp and it opens far enough. I have found that in salt water they start getting sticky after a few seasons, or what happens is that a piece of sand, or a rust flake gets stuck in the seat and then you have the opposite problem, the engine never warms up fully.
Lastly, exhaust elbows gradually start to clog due to rust flakes, but in freshwater that is not much of a problem, here in coastal salt water you have to figure on replacing them every 5-7 seasons. The manifolds can last longer, but the exit end of the elbow is what suffers the most. Here are an old pair of OMC one piece V6 manifolds from my boat after 5.5 seasons here in the Long Island Sound region....One is used, the other new. I used these for 15+ years here in salt water but changed them after 5-6 seasons. Since they are long since NLA, I switched to the later OMC/Volvo 2 piece exhaust system.
Old Batwings.....
OMC batwings after 5.5 seasons vs new.jpg

trial fitting the 2 piece system used by OMC & all fit perfect & no leaks....

OMC-Volvo exhaust conversion.JPG
If your oil pressure is good, I don't see any reason for a 20w-50. Even 90 degree water should keep the engine cool enough if the cooling system is in good condition. I used to run (previous boats) 5w-40 diesel rated oil with good results. Now I am going to run Triax 25w-40 full synthetic marine oil in my boat at Havasu. Never looking back. By the way, last time I looked the Quicksilver 25w-40 oil was diesel rated as well.
I'll second (or third?) the use of M1 15w-50. That said, I use Valvoline 20w-50 in my antique inboard w/ a Merc 350 because I use a mix of avgas including leaded fuel and leaded fuel and synthetics are a no-no...