A plastic sheet actually helps a tacky rubber keep restore its tackiness. Any plastic sheet is fine. The plastic sheets from report covers will work. And any plastic sheet will work. What Der_Echte said about the plastic you wrap food with, it will work. So will a plastic bag for storing food. But the firm plastic from a report cover is what I like to use.
If your rubbers are still tacky, adding oil is not really something you should do though. When they start losing their tackiness, then adding some oil will cause the rubber to restore some of the tackiness. I would use mineral oil or baby oil rather than olive oil or vegetable oil. Vegetable oils will do something. But mineral oil will actually be better for the rubber. WD40 also works.
One more thing about oil: don't use too much: a certain amount added to the topsheet will help restore tackiness. A certain amount added to the sponge will cause the sponge to have a little more responsiveness for a few weeks. But too much oil can cause the glue where the topsheet is attached to the sponge to dissolve and then you get bubbles. You can get this from putting too much oil on the topsheet or the sponge separately. If you do both, you run a higher risk of bubbles in the topsheet. Also, don't add oil too frequently. More than once a month, after a while can cause the rubber to stop performing and to weigh too much from how much oil has been added. In the short term, oil seems to make the rubber more elastic. In the long run, oil causes the rubber to oxidize faster and become less elastic faster. So it is something to use on a topsheet when your rubber is already losing grip. Not when it is still performing well enough.
While just cleaning, the way Clu37 suggests will be much better than not cleaning, the plastic sheet actually does help tacky rubbers keep that tackiness. It has to do with how the tacky rubber will stick to the plastic.
One year: While the tackiness of the topsheet is part of what makes a H3 rubber tick, so is the sponge. Rubber molecules oxidize and break down over time. A year is a good long time to use a rubber. Longer than a year is kind of a surprising amount of time. I know Der_Echte has had some rubbers that he used for over two years. But as the rubber oxidizes and breaks down, it becomes less elastic and does less stretching and rebounding. So even if you are thinking the rubber is providing enough grip. It probably won't be doing all the things a rubber should do to help you spin the ball.
Not everyone can tell when a rubber is actually dead. But when a decently high level player picks up your racket and hits with it and says, "your rubber is dead and using this could mess with your technique. You should think about getting new rubber," they may know what they are talking about.
So the actual answer to your poll is that, whether you should add oil to your topsheet or not depends on how the rubber is actually performing currently.
And, it sounds like your rubber still grabs to ball well so, NO, you should not use oil yet.
