Mobil oil is picking up the tab for the Public Broadcasting Service's presentation of Robert Gravess' "I, Claudius." The series is so much finer than any of the dull dramatizations of trash fiction on commercial TV, you do yourself a disservice if you don't tune in - always providing, of course, the government didn't stick your community's noncommercial station with an unwatchable permanently snow-shrouded UHF channel.

You need not feel too indebted to Mobil, however. There's politics in the Petroleum Broadcasting System, as you increasingly hear PBS called. Oil company sponsorhsip of anything appealing to the upper-middle class is quite heavy. Note the Exxon commercials on "Meet the Press" and "Face the Nation" with "I, Claudius," the monster oil corporation has gone so far as to buy a double-page ad in The New York Times magazine section - upper-middle class again - with a pull-out program guide that includes, would you believe it, a reading list! Lordy, Lordy, the next thing you know they'll be cleaning your windshield again.

No matter how good "I, Claudius" is, and it is very good indeed, it is disconcerting to learn that Herbert Schmertz, Mobil's vice president for public relations, appears to have been the person who ultimately 12-part series would air in the United States. As is so often the case with adventuresome television, this package was done in television where it has been well-received.

We can't say for sure that if Joan Sullivan of Boston's WGBH-TV, who has handled the American presentation, has been turned down by Schmertz the series wouldn't have aired. We can no more that than we can say for sure the cuts - they prefer to call them "edits" - made for the American showing were made with Mobil in mind. ("I, Claudius" contains more than a minimum of sex and gore, but readers of Suetonius will tell you that next to the Emperor Caligula, Charlie Manson looks like the amiable, fat monk in the Zerox commercials.)

It can't be said that Mobil has abused its power in this instance, but should it have such power over noncommercial television?

It appears that what PBS has become is a very inexpensive way to do institutional advertising to a group with premium demographies: high income, high status, high-educational types who've been bored by commercial TV too often to watch it any more. Advertising on noncommercial TV is very cheap. Not only is there no profit, but taxpayers and individual small donors pay 99 per cent of the freight. Then a company such as Mobil can move in for a couple of hundred grand and get the credit.

We might ask ourselves what's the reason for noncommercial TV's existence? It is only to get ads offs the air? Even in this, noncommercial TV hasn't been altogether successful. Have you noticed the amount of promotional clutter on PBS stations?

There are all kinds of goodies on noncommercial TV. Even a series such as "Best of Families," which would have been better named "White Roots," is, in historical accuracy, superior to the black "Roots" from which it derives. But if programs such as "Visions" and "Nova" often have first-rate installments, PBS still emits the odor of commercialism and commercial control.

The Carter administration has asked Congress for a large increase in noncommercial TV appropriations. Let this be done, but with the provision that Mobil and the rest of the oil altruists clear out. Corporate America already runs three networds. That should suffice.