There are 2 ways to view the money.
Pre US and Post US.
Post US: If US will build say an extra 2 fighters and a carrier with the money it will come out post Pearl Harbor
Pre US: If they gave the same money to UK/China its used to do damage (hopefully slowing) the Axis, If UK builds say 2 extra fighters what’s the difference, aside of more time those planes can hinder Germany & Italy.
I would love for US to do max lend but in all honesty that’s not practical, at least not all 8 turns as the US needs to build some stuff vs. Japan to slow there romping in the pacific till the US has there 125 to start building a real force against Japan.
So my thoughts are for the US turns:
1) (25) 2 factories, 10 UK, 3 China
2) (25) 2 factories, 10 UK, 3 China
3) (25) 2 factories, 10 UK, 3 China
4) (25) 2 factories, 6 UK, 3 China, 1 fighter
5) (27) 3 factories, 2 (UK or China), 1 fighter, 1 troop
6) (27) 3 factories, 2 (UK or China), 1 fighter, 1 troop
7) (27) 3 factories, 2 (UK or China), 1 fighter, 1 troop
(27) 3 factories, 9 to spend on carrier, battleship (or some long term ships that you will have come out on turn 12 with a large force of other navel stuff)
Now this obviously should be adjusted, if UK is doing well with the battles or German subs were wiped out with minimal damage and UK doesn't need 10 then toss more to China or build more stuff, if Japan isn't messing with China at all, they wont need any funds.
I'm also not sure on the turns how the fighters will work, but I think its important for the US to have at least 2 extra fighters on Hawaii so there is a potential of costing Japan more in Pearl Harbor (or maybe saving your battle ships there).
Why bother giving 3 to China or 10 to UK? If China has 1 more troop out there that can help hold a territory it takes 1 production from Japan (per turn), or if the unit hits it costs Japan 3 (to replace that troop) as well as Japan has as very limited ability to build troops and they need them for the island hopping. Same with UK, if UK has the money to put pressure on Italy, maybe it will slow them 1 turn coming into the war or might cost Germany more in France.
At the same cost, a unit now who can do things vs. a unit later who isn’t in play seems like an obvious choice.
You can also think of it this way, if UK had saved say 5 every turn for the first 4 turns (20) and waited for 4 more turns before spending it would seem foolish.