A juki 241 manual will do just fine , I sew all day every day, and I check, oil my machines about every three days, or Listen to the ole Girl.When she is properly lubricated it has a smooth sound, when she needs oil she will get louder or chatty.If you lean your head back there is a main oil reservior in the bottom. Fill it up to red line on glass. Not all oil ports are marked. Look for journals with oil holes in them. These are different from the red marked main journal / moving parts oil holes.If you sew every day / all day put 2-3 drops in all Red Ports. You can cheat in the head, Pull back the face plate and spray the whole inside with a spray lubricant.
I clean out the machines every two weeks. Lean back the head and use carb cleaner with spray tube to get in tight places to wash out fuzz, thread and dirt. Place paper towels or rags in the pan to catch the dirt, crud, and carb cleaner. the cab cleaner will evaporate quickly and leaves it clean with no residue. Warning!!!, do not spray card cleaner on the site glass or any rubber parts as it could dissolve them.Use a Snorkel bottle of machine oil available in most hardware stores. These have a retractable tube about 6 inches long. You pull out the tube and it allows you to stick it down in the red Ports till you feel it hit some thing, When it touches give it three drops. I would reccommend getting a sewing machine mechanic to check out your head and adjust everything, he can then show you all the fine adjustments, lube points and the minor adjustments that the operator normally does when operating the machine. It would be the best 75-100 dollars you ever spent as it could save your 1500.00 machine. Keep it properly oiled and it will be making seat cover for your grand kids