Alternatives to single grade oil in older engines?

[Rolo]

According to the manual my 1975-6 Ford Falcon engine needs single grade 30 oil there is no mention of any other alternative multigrade oil one could use (probably they didn't even exist in the early 70s) and the generally "accepted wisdom" now is that one should use multigrade 20W50 but when I have asked in the past why we should use an oil that's almost twice as thick as originally recommended the usual reply is a not very convincing "because your engine is now old and the tolerances aren't so tight anymore", but says WHO?

The engine design may be old but what if I this engine had just been reconditioned and everything is just as tight now as when the engine was when new or only a couple of years old?

Do Ford engineers say that their new cars that now use 0W30 or 10W30 oil need to start using 10W40 and then 10W60 as the years go by?

If that's not the case, which I'm sure it isn't, then why shouldn't owners of older jalopies still benefit from newer oil technology by using an oil that will be much THINNER than a single grade oil on starting (which should be VERY desirable in cold starts in cold Winters (*)AND HOT SUMMERS to get the oil flowing as quickly as possible and still be *JUST AS THICK* as it needs to be (as recommended by the car manufacturer) when the engine reaches normal working temperature, which is a few degrees either side of 100 degrees Celsius - 93 to 105) and that's a fact that doesn't change with the seasons which is why thermostats and cooling fans are needed.

Should I use 10W30, and if not why do you think I shouldn't and what would your recommendation be?

Is there some other reasoning behind the "20W50" argument that people failed to explain to me or that I have failed to understand?

(I'm just an amateur and I know a lot less than what most people may believe if they judge my knowledge by reading just one of my posts)

(*) A "cold start is defined as the starting of an engine that has been turned off for at least 4 hours and even if it's Summer and 40 degrees Celsius in the shade when you do it's still defined as a cold start because it would still be a good 60 degrees below normal working temperature.

[madmark]

mate i run my essex v6 engines on 20/50 anything older type this seems the best type oil IMO

[Rolo]

mate i run my essex v6 engines on 20/50 anything older type this seems the best type oil IMO

What grade oil does the manual say you should use?

[Sundowner]

I use Penrite HPR40 for the 250 in my van and cortina race car always have always will
I only use Penrite oil