"Use 30wt oil"

Are you talking about using that in the gearbox? Good choice, especially with an overdrive. Straight cut oil lasts an eternity with helical gears. Use the 80W-90 gear oil for hypoid gears only.

"The drain plug uses a copper sealing washer and it really should be changed too, but for some reason these are hard to find."

They are hard to find because they never need replacing. It's COPPER, just anneal it once every so often.

"Use.....an oil filter that has an check valve and a relief valve."

How often have you seen one without a relief valve? I use those for bedding in a cam else if you knock a lobe off, you can fill the filter with trash, then the relief valve cracks open & lets trash through the bearings.