of course it will
remeber this...the less difference there is between "weights" the longer that oil will last before breaking down
10.40 will last noticeably less at its correct "weight" before say a 20-50 or even a 15-40, this is visible even to the naked eye....
thats all Im going to say now...
10-40 is stocked almost anywhere
on an atomic 4 gas inboard any car 10-40 is ok, even moyer marine recomends this with a dash of marvel mystery oil
delo 400, rotella t are really good oil and I have noticed from using them in many vehicles from offroad motorcycles to old 4x4s that they last and keep their viscocity well
on old engines you neeed that as tolerances losen up as you age...
synthetics are the antitheses if viscocity retention over a period of time...they do however tolerate EXTREME HEAT better and they dont break downs as HARSHLY or quickly if you will
sooooooooooo if it where ME with your engine go for the 15-40 dino stuff and change regularly before any breakdown happens
and stick to that oil
good luck in your decision.
ps. minnewaska is absolutely right, DO NOT STRAY FROM WHAT THE MANUFACTURERS SAY TO DO as a general rule...unless that advice is obsolete...
this applies to oil technology, fuel, additives etc...this is a common question when restoring vehicles for example