Where to use T4 15W-40 in a 10W-30 World?

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I am retiring an old Craftsman leaf vacuum with a 4.5 HP Tecumseh that had been running on T4 15W-40. This leaves me with 2 quarts of unneeded oil. My outdoor fleet is now almost exclusively OHV powered with 10W-30 oil recommended. The exception is a 1985 vintage Troy-Bilt (real one - not MTD) with a 5 HP Briggs flathead. This puppy calls for SAE 30 but currently runs T6 5W-40. I have no intention of putting the 15W-40 in there because this machine does most of its work at around 40 degrees at the beginning and end of the gardening season. Maybe I should rethink that? So, should I just use it as top up oil in whatever needs it, or is there a better alternative?