Oil recommendations

[Cooler]

edw - I would not use Rotella diesel in gas engines. I use Quicksilver 25-50w. It is a blended synthetic, offering more actual lubrication than 40W oil. I am not going to start my engines at 25F degrees anyway. The water gets way too hard ( ice ). Oil no longer breaks down due to time, just actual use. I have 40 hours on my last oil change, and it still looks like new oil. No discoloration from heat. I have Crusaders. er

[KandJ72]

Why you ask... Because it works. If you think about it most engines being used by people on here are cruisers. Made for torque and steady sustained running, very similar to the conditions that diesels are built for. They're generally only team in a warmer conditions and need to hold up in hot engine compartments. It's a good solid pull that protects very well at a much more affordable cost than alot of the mfg branded oils. IMHO that is.

[tomschauer]

Those 7.4's will be happy with Mobile 1 15/50.

[tomschauer]

If you switch to Mobile 1 or most likely any good synthetic oil, after your next oil change you will notice the oil is unusually black and nasty. After every subsequent change it will look nearly as new.
It will break up all the sludge the dino oil has left behind.

[Mississippi Voyager]

Hi, I'm new here, but a lifetime boater. I recently did an absurdly detailed study of oils for my 1995 Crusader 350XL's. This will directly apply to Mercruiser models from the same period. GM changed these engines in 1996 from flat tappet cams to roller cams. Prior to 1996 the 350 XL had flat tappet cams and the 350XLHT Crusaders had roller cams. I'm not sure what Mercruiser had as an equivalent model to the HT Crusader. If your engine has roller cams this oil discussion does not apply.

Having flat tappet cams means your oil needs to have zinc plus phosphorous (ZDDP) to lubricate the cams. Most modern oil has removed this additive because it is not compatible with the Catalytic converters on cars. All racing oil and Diesel oil has this additive. Rotella Diesel oil is what I now use after all of this research. I went with the full synthetic (T6) 15w-40, but the semi synthetic (T5) or the regular version (T4) would also be OK. T6 is only 22 bucks per gallon at Walmart, so when I compare the cost of 2.5 gallons needed for my twins to my overall operation costs, it is insignificant. While this is marketed as a Diesel oil, it is fully compatible with gas engines that do not have catalytic converters.

The owners manuals for the older engines specify single weight oil, because that was the best available in the 80's and 90's. They later changed this spec in the manuals to 15w-40 as the recommended viscosity. Shell Rotella 15w-40 has just the right amount of ZDDP and the proper weight for our GM based engines with flat tappet cams. I found online compiled by a hot rod racing group of what oils have ZDDP. ( https://www.autorestomod.com/oil-zddp-list.html ). Or google Auto Resto Mod ZDDP. Most of the hot-rodders engines are flat tappet. If you google the subject "Flat Tappet ZDDP"you will come up with lots of info, but most of this is not generally known by the average boater. If your engines are 1996 or newer, you have roller cams and need not be concerned with ZDDP.

[Mississippi Voyager]

Tomschauer,

You are correct, sir. For flat tappet engines (GM 1995 and earlier) Mobil 1 15w-50 is one of the variations of Mobil 1 that has the correct amount of Zinc and Phosphorous for our engines. Most of the Mobil 1 variations have 650 to 900 PPM (Parts per Million) of ZDDP, which is considered to be too low for flat tappet engines. Optimum is around 1200 PPM. 3 of the 37 variations of Mobil 1 have the right amount of ZDDP.

Mobil 1 15w-50 has 1200 PPM Phosphorous and 1300 PPM Zinc. That is the closest Mobil 1 makes to the ideal of 15w-40 with 1200 PPM ZDDP. This link will take you to a download of the PDF for all Mobil 1 oils and their ZDDP content: https://www.mobil.com/lubricants/-/medi ... 396657AC09

Mobil 1 15w-50 would be my alternate choice after Rotella T6 15w-40 1200 PPM ZDDP. (Again, for flat tappet cam engines). The Rotella is easier to find in my area and a little closer to the perfect spec.

[SunsetSteve]

Looking for certainty in this terrifying world of oil decisions.
Manual for my so-far-not-used-by-me 1983 Riviera (laid up for winter in my yard) with a pair of Crusader 220s (300 cid I believe) calls for 10W30.
I still have 2x4liter jugs of T4 15W-40 Rotella from my Albin diesel days. As I stated on another thread here - the other Rotella thread - my thought was to buy a tonne of 10W30 from the marine store to do an oil change in the spring.
Back and forth she goes - o now rethinking using the Rotella . . . hmmmm
(Then to find the appropriate oil and fuel filters.)
Most info on the net addresses more modern versions of the Crusaders.

[RGrew176]

My mechanic uses a synthetic blend oil. Can't tell you what brand or weight as I have never been there when he has changed the oil. 1981 5.7 Crusaders. Seems to be working out OK.