Mobilmet 766, replace periodically or just top up?

[AAA]

We have a machine that runs Mobilmet 766 cutting oil.
Up until now we have just topped this machine up with cutting oil as necessary, as it left on parts.
It is never run to the point it smokes or anything.
It was mentioned to me that this cutting oil should be completely replaced on a regular basis as the sulphur element "wears out".
We haven't really noticed a loss in performance, but the base oil in the machine is close to 7 years old now, so likely I don't remember what it cut like when new.

Obviously (because they want me to buy more) Mobil recommend replacing it, what's been the experience of others?
Machine runs a 50 micron filter, and oil passing back to sump passes through a neodymium magnet, that pulls out any metallic particulate.
Amount of oil is about 40 gallons.

[Steggy]

We have a machine that runs Mobilmet 766 cutting oil.
Up until now we have just topped this machine up with cutting oil as necessary, as it left on parts.
It is never run to the point it smokes or anything.
It was mentioned to me that this cutting oil should be completely replaced on a regular basis as the sulphur element "wears out".
We haven't really noticed a loss in performance, but the base oil in the machine is close to 7 years old now, so likely I don't remember what it cut like when new.

Obviously (because they want me to buy more) Mobil recommend replacing it, what's been the experience of others?
Machine runs a 50 micron filter, and oil passing back to sump passes through a neodymium magnet, that pulls out any metallic particulate.
Amount of oil is about 40 gallons.

The oil does deteriorate from use and also from atmospheric exposure. It should be dumped and replaced every couple of years.

[Harold_V]

While it may be true that the oil may degrade with time, it would be my opinion that it may not need replacement, assuming it is performing to one's satisfaction. Said another way, if the operation goes without issues, why fix something that isn't broken, especially when the oil companies are now selling oil at five times its price not too long ago.

H

[Russ Hanscom]

How much do you add periodically? If for example, you are adding 10 gallons a year, then you are effectively replacing the oil over a number of years.

I would tend to agree that if you have not noticed a loss of performance, then there is not a big need to replace the oil.

[AAA]

Thanks guys.
Harold always good to hear from you sir!
Cost - this is why I'm posting, the cost to replace it all would be excessive, I think I'm going to go with the - "If it ain't boke, don't fixed it" theory.

[mcostello]

From what I have seen on the 'net, the Navy never changes oil on the big ship gearboxes, They use a centrifical filter and just top up what might leak out.

[spro]

AAA. At some point, someone is going to ask about 40 gallons for 1 machine. Serious biz there, to put it lightly. Our newbies and home shop and experienced pros are not going to run away screaming in the night if you post a pic. It doesn't have to be yours, only similar.