I've had mine from brand-new. It came with a little bottle of oil. It's a work horse. Clean it regularly with a lint brush or gentle vacuum. You do need to service it now and then. I hope mine never dies. It's my backup in case my new, computerized machine decides to bite the dust.

These models are lubriated, but meant to be done during a service. The electronic circuit board doesn't need oil, but all moving joints, gears, hinged parts, where metal move against metal, could do with some kind of oil. It will make the machine turn run lighter and smoother. The trick is to get the covers off to reach the part, and to oil sparingly, neat with out excess, but enough to reach the inners of noocks and crannies. Except for the step motor and no cam stack, the moving parts are much the same as on any new or vintage sewing machine. The only parts not taking oil can be nylon or loosely clipped together parts, but actully, most of these to recieve some kind of lubtication during a service.

What kind of feet do these use? Low shank, or what?

Accessory storage compartment
 

As long as we are on the subject of mid 80's machines, I'm always on the look out for this series at thrift stores.
The model 6235 has a complete set of attachments in a built in storage compartment on top of the machine. Here is a picture of a model 7050 to show what I'm talking about. The model 9131 Has the same set up.

oiling a Singer 6233
 

Hello. I'm new to this forum. I bought a new Singer 6233 machine in the 80s in New York City. I used it steadily till about 2000. I gradually purchased additional machines and used the 6233 less and less. I never oiled it but it was always reliable. About two years ago, it started skipping stitches. I brought it to a qualified Singer repair person. He said it would cost me $150 for him to replace a computer system inside the machine. I told him that I would just pick up the machine. From then I have used it for quick zig zags. But today I used it for some straight stitches and sure enough it skipped stitches again. Examining it I saw there were holes for adding oil in several accessible places. So I got out the manual and was surprised to see that, just as mentioned above, there was no instruction for oiling the machine. So I came downstairs and searched for answers on my computer. Low and behold I found this forum. I'm going to go up and oil those holes and see what happens. Why not? I will let you know.

"Self-lubricating" machines have been around since the 60s, at least for Husqvarnas. If the owner's manual doesn't have oiling instructions, it is probably expected to be part of periodic dealer maintenance. That said...