Number 8 size hose (1/2") is normally used on most four cylinder Lycomings (320/360). Using number 6 hose (3/8") will give you a pressure drop of about 26-27% compared to what #8 hose would give you for a typical 1-4 foot length of oil cooler hose. Using #6 hose might be OK for an O-235, I'm not really sure, but I'll bet I'm not the only one who'd recommend using #8 oil cooler hose instead.
Also I recommend using professionally-fabricated, stainless-braided and silicone-firesleeved teflon hoses with swaged fittings instead of legacy Stratoflex 111 / Aeroquip 303 rubber hoses for all aircraft oil cooler and fuel line plumbing under the cowl. The teflon handles heat much better, lasts practically forever, and the swaged fittings pretty much never spring a leak. The thought of an oil or fuel fire in-flight due to a hose leak is one of my worst nightmares.