can full synthic oil be used in a 1999 cabio

You can use synthetic in any vehicle but I would never go past 5000 miles even with it. Any type of oil breaks down and loses viscosity after a while and needs changed at regular intervals. I always change mine between 3 - 4000 miles.

While it's true that synthetic oil is more durable and doesn't break down as quickly (into the acidic organic compounds that Dino-oil is composed of), it still has to suspend all the particulate byproducts of combustion and engine wear. The benefits of synthetic are that durability and better start up protection, especially in cold weather where the synthetic remains fluid at much lower temperatures. Conversely, dino- oil tends to condition seals and gaskets, making them swell slightly...so, if you then convert to pure synthetic oil, those same seals may shrink a bit causing the leaks so many are concerned with upon changing from conventional oil. In summary, if your engine is high-mileage and you've been using dino-oil for most of it's life yet you want the advantages of synthetic lubricants, I strongly recommend a synthetic blend while retaining the oil change interval recommended for the conventional oil. That way you're changing that filter which collected the particulates and you're keeping those seals happy....all the while protecting your valuable engine internals.