Steinbeck didn't just prefer pencils. He treated them like instruments, buying them by the dozen and burning through a startling number every working day. He was fussy about the exact one.

The Direct Answer
Yes. He wrote longhand in pencil, often a dozen or more a day.
He sharpened a full tray each morning and worked through them, per his own letters in Journal of a Novel.

Round, Not Hexagonal

Steinbeck wrote everything in pencil, and he was particular about which one. He drafted longhand, page after page, and he didn't move to a typewriter until much later in a manuscript's life. That meant the pencil was in his hand for the whole working day, so its shape stopped being a small detail. He wanted them round rather than hexagonal, because a hexagonal barrel left ridges in his fingers after he'd held it for five or six hours straight. The flat edges bit into the same spots over and over, and by afternoon his hand would ache. A round barrel rolled under his grip and spread the pressure, so he could keep going.

His early favorite was the Mongol 2 3/8 F, which he called 'quite black' and praised because it held its point. He bought them four dozen at a time. Each morning he'd line up two or three dozen freshly sharpened pencils in a tray on his desk and write until each one dulled, then reach for the next without breaking stride. He worked through a surprising number in a single sitting, sometimes a dozen or more, which is how the habit got its reputation.

He had a tell for when a pencil was finished. When the metal ferrule that held the eraser started touching his hand, the wood had gotten too short and the balance felt wrong, so he'd toss it and grab a fresh one. He called this fidgety pre-work ritual 'pencil trifling,' and he was honest about what it really was. Lining them up and testing each point was partly preparation and partly a way to stall before the hard part of the day began.

The Blackwing He Loved

Later he found the Eberhard Faber Blackwing 602 and never looked back. Its soft, waxed graphite let him write with almost no pressure, which mattered enormously over a long day at the desk. A harder lead made him press, and pressing wore out his hand faster. The Blackwing laid down a dark line for almost no effort, and it didn't crumble or snap the way softer leads usually did. For a man who measured his pencils by the dozen, that combination was close to perfect.

I have found a new kind of pencil, the best I have ever had. It is black and soft but doesn't break off. They are called Blackwings and they really glide over the paper.

John Steinbeck, in a letter

He kept an electric sharpener on his desk and ran the whole tray through it most mornings, which is part of why he could blow through so many in a day. A dull point slowed him down, and he hated being slowed down once the writing was going. The pencils, the tray, the sharpener, and the dawdling were all one system that pointed in a single direction: get the hand moving and keep it moving. For the rest of the picture, see the full John Steinbeck writing routine.

What You Can Steal