Hi Guys,
Oil, what a slippery subject to pin down. (could not resist the pun, but I am seeking help for it.)
Modern motor oil is usually high detergent, which is there to wash the crud about and keep it suspension till the “micronic” filter removes it.
Older engines with only gauze filters have to use a low or non detergent oil, so that the crud can settle in the sump, until it is washed out with a flushing oil. Well that is the case in an ideal world. Multi grades are almost exclusively high detergent, and are great in modern car engines, older bikes and cars it is not so good unless a modern filter can be put in circuit.
Mono grade oil is available as a high or low detergent, depending on application. Some modernish bikes, like my seventies BMW, if used at high speed in a hot climate use a straight 40 grade HD oil. (Although I had a big argument with a pump attendant in Italy over the matter, he would not let me buy any, well to start with that is!!!)
Low detergent mono grades are used in older machines, be it bikes, cars, boats, tractors etc. where an oil with a bit of “body” is needed.
So if you are going to pick 30 weight oil for your lathe, choose one with low or no detergent. These are most likely to be found at an old motor bike shop, an agricultural suppliers or a boat centre, where they are still in vogue. Modern car shops usually only sell modern multi grades, which is perhaps not what you want.
There is a good case for saying any oil (with in reason) is better than none, but the right stuff should always be sought.
It should be remembered that the point of a multi grade oil is that you can use the same stuff all year round, Although the engine temperature is regulated by the thermostat when running, you need a thin oil to let the poor old starter motor work and let the oil pump do its job on a cold morning. The “long chain molecules ” of a multi grade are to provide the protection from metal to metal contact that wears an engine, but the stuff is as thin as p*ss water, when hot. On old air cooled engines using a straight oil of the right grade can make a notable difference to the rattles!!
Before some oil company chappy chimes in, I know there is a lot more to modern oils, but we are talking about lathe use here, not turbo charged Vee12’s.
chris stephens