Hydraulic oil question

I am not an engineer, my tug does not have an engineer, we have an anti-engineer.

It’s a small tug 65ft and I would like a recommendation on hydraulic oil to buy for the steering system. It has not been changed since we have owned the boat.

It’s a home or yard built system with a small tank, two pumps, two rams topside, and some valves and piping. I do not see a filter in the system. When shopping I see different viscosity like AW32, AW46 etc

Can someone explain in layperson’s terms the difference and recommend something?

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First off hire an engineer, they can be shoreside but get a person with real mechanical know how.

Secondly your steering gear is one of your vessels most important systems, don’t take internet advice.

Call whoever built your steering gear and ask them what they used (blending hydraulic oils is bad)

While you’re at it change your filters

I was more interested in the viscosity part of the question. I am attempting to “profesionalize” the boat for lack of a better term, but it’s an uphill battle. My owner who rides the barge buys oil at price club by the case when on sale and buys gear oil at Autozone.
He buys Shell Rotella 15w-40 and puts it into everything, I have intervened on behalf of the 3-71s and purchased the correct oil for them.

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Just some thoughts in addition to @DavidMT advise (which you should listen to) , but based on your description of this operation in general I’m going to assume you don’t do any sort of oil sample testing. If you can’t find any info on the original maker, it might be worth sending a sample to a testing lab to find out what viscosity you are running now. If the system functions without issue now it makes sense to stick with similar oil knowing nothing else.

There are other things to consider too, like temperature rating of the oil. It should be easy enough to take some temp readings during normal operation.

No matter what, if you don’t know what’s in there you’ll need to buy enough oil to flush it. Out with the old, in with the new, run it, cycle it, drain and fill again. If you have a filter, change it both times, if not, you should strongly consider getting one in the spirit of your effort to “professionalize” the boat.

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That’s funny! Sounds like he should be ran off.

Good chance its 46. Taking oil samples is some good advice. Shouldn’t be too expensive and from now on you should make a habit of it. Regularly take samples on everything. It kinda sounds like you guys may not do things per the manufacturers reccomended hour interval so maybe doing it every month or two may be good future practice.

If plumbing in a filter to the hydraulic system isn’t an option, another one would be a filter polishing cart to cycle the oil thru every now and again.

You could just say fuck it and go get a job with a more professional operation too. That may be easier!


I prefer the mom and pops. If I went to a bigger outfit they would just make me work more. The anti-engineer is juiced in, but he is on my list of things to remove from the boat.

I have a new person hired trying to get a handle on things but the other guy is “in the way” of progress.

I just sampled the main engines. I never thought of doing the hydraulic oil, that’s easy enough to do.

I’m going to borrow and make good use of your term: “anti-engineer”. It’s fantastic. Thanks.

First of all, I’m not an engineer. So take what I say with a grain of salt.

You described a simple “homemade” hydraulic steering system on a small boat. If it’s “homemade” it probably was not “designed” by the “anti-engineer” with any particular oil in mind. No need to over think it.

If the oil is red, it’s probably Type A automatic transmission fluid which is common in small systems, especially in cold climates, and especially when the oil is bought by the quart at Autozone. If the oil looks clear or has a greenish tint like new engine oil, it is probably hydraulic oil. The viscosity should be chosen based upon the operating climate. Unless the climate is very cold or very hot, the viscosity won’t matter much.

NAPA and similar places carry “tractor” hydraulic oil in 5 gallon buckets. It is a higher quality, and higher priced, oil designed for torque converters in tractors. In a tractor the torque converter and the auxiliary attachment hydraulics share the same oil. A torque converter really needs this high quality “tractor” hydraulic oil.

I generally use “tractor” hydraulic oil in every hydraulic system. It’s the best regular hydraulic oil that is readily available. The extra cost does not matter in a small system.

In complex, high pressure, high value, hydraulic systems, I use expensive synthetic hydraulic oil that meets the equipment manufacturer specs. I prefer Amsoil products. That would be overkill for your simple “homemade” hydraulic system. The owner won’t want to pay for it either.

When dealing with a small boat and a cheap owner, practical compromises must be made. Save your ammunition for the important items, which hydraulic oil to use isn’t worth using much ammo.


read slower guys, his ‘anti engineer’ isn’t going to pay for oil analysis and probably isn’t going to buy the right oil till all the seals have to be replaced. Neither does he see any filters in this system which is hard to believe but it could just be a strainer on the pump intake?
We don’t know how many hours the system has on it so how it is performing is just a guess but it’d probably benefit with a step up in grade of oil.
I wonder what sort of oil his 3-71’s were eating? I just rebuilt one and am NOT skimping on oil there!

I have a sample from the hydraulic system and will send it out after this hitch. I have the anti-engineer changing the oil in the 3-71 gensets to sae30w that I purchased.

We are making some progress.

In the wheelhouse the steering seems slow and getting slower. I immediately asked the guy to change the filter and he said there wasnt one. When I inspected I didnt see one either, but I havent traced out the whole system yet.

It’s the type that has mechanical linkage all the way to the wheelhouse sticks. We dont have the sticks in use but, I tested them and they still work.

The recommended lube for that engine is DELO 100 SAE 40. Use SAE 30 if you are in a cold climate.

ya, delo… i’m using 400 in my diesels. anything else has to be special ordered out here in the sticks but detroit is happy with it.

Detroit 71 series run fine on just about any oil. Theoretically, they should have the proper detergent and additive package, but in practice it’s not all that important.

I use Shell Rotella T for my Cats, but also use it the Detroit’s, rather than carry another flavor of oil. I’ve done it for years.

Rotella T is very good oil. When you first put it in an engine, it may smoke more, but after awhile it will smoke less than it did before. It tends to reduce oil consumption.

Detroit’s all burn and leak oil.

In a modest size engine that does not burn and leak oil, I like to use synthetic oil

Detroit oil needs to have zinc, not many oils have that anymore

(maybe slightly OffT)

Meow… that’s a perfect body lotion !
Mice and big dogs become unable to detect our strong Cat odor.