I have a 68 Bonneville converted to a Tiger top end, running a Routt 750 kit, Jomo 15 cams, 1978 T140 five speed, lightened stock clutch, 3 phase alternator...no battery, Black Diamond valves, Ampco 45 guides, light valve train, tall final drive gearing. I used it as my transportaton (I didn't even HAVE a car license) for over 10 years and put over 85,000 miles on the engine after the rebuild in 1995. It is currently still doing duty in a friend's bike as I rebuild his TR 7 engine for his chopper while I redo the frame and bodywork (original faded paint.)
Here is what you do.
Run the bike to get the oil and engine hot.
Drain the tank and the crankcase of engine oil. The drain plug for the case it the angled one with about a 13/16" hex. The drain plug for the tank is 3/8 BS. Change the angle of the bike to get all the oil out of the tank and engine. With the plugs out, kick the engine over to remove all the oil that you can from the lines and passages.
Put the drainplugs and spark plugs back in and put a quart of full synthetic Amsoil or Royal Purple (or even Mobil one NON-ENERGY conserving) 10W-30 in the tank. Run it for fifteen minutes or so....a trip to the beer store is about right. Drain while hot, following the previous instructions. This time, fill the tank with three quarts of the synthlube.
Drain the primary case by removing a hard to access plug (1/4BS) at the bottom rear of the case (not in the cover....that is a level plug). You may have to remove the left peg to get access, especially if you take this opportunity to also adjust the primary chain....there is a special tool for this, but it can be made from a bolt ground to a screwdriver tip. If you want instructions on this, ask later.
This will drain slowly, because the adjuster is in the way, but give it a half hour and you should be done. Plug back in and all the other stuff back on and fill the primary case through the slotted plug on top of the primary case, rear of the cylinders with 1/3 quart of synthetic ATF. Even Dexron is better primary oil than the factory's recommended 20wt oil. ATF is designed to work with wet clutches to provide good operation of the clutch and good lube of the gears.
Drain the gearbox oil from the plug in the bottom of the case which as a second smaller bolt in the center. It is close to the right frame tube. It is also a BS hex. I think it's 1/2BS. Don't remove the long plunger housing with the rounded bottom. Rock the bike to get the most of the oil out. It will stink of sulfur compunds, as it's probably hypoid 90 wt. Put the plug back in and fill with a half quart of full synthetic 75/85 or 75/90 gear oil. The kind that goes in differentials. Your bike has hobbed, unlapped gears and shouldn't use motorcycle or engine oil in the gearbox. It must have high pressure additives. Hence the stink. I would change this oil out after about 500 miles with the second half of the synthetic. Now you are lubed up for a good long ride. As long as your bike isn't running really rich or has some mechanical problem, and if you aren't hotrodding about, you will be able to go 8K miles between changes.
I will be pulling my engine down and taking some photos of the results of it's life with synthetic lubricants. But I have had flawless reliability with it. I even removed the oil filter when, after doing a postmortem on a couple of filter elements, I found no metal particles. So the last 40K or so has been run sans filter, like most old bikes, but has been subjected to the travesties of city riding and high speed touring on the Big Island, and everything in between.