R1150 GSA oil weight; winter to summer

machinery lubrication showed a graph how oils act thin-thicken outside of the spec 40C + 100C they showed how real synthetics which are few + costly thin less in extreme heat + thickening is even worse in the cold! if it fits $$$$ a motorcycle oil like Redline 15-50 is my fav + can be left in service longer defraying the xtra cost.
If you are running a 15W 40, you might as well run a 15W50 instead (or even a 20W50 full synthetic). For cold weather starts, it's primarily the lower number of the grade that is important. There are also some 10W59 oils available and because of the dry clutch and separate transmission, you do not necessarily have to use a motorcycle specific oil.

What is the lowest temperature that BMW considers OK for use of 20W50? A 20W50 may be suitable for winter where you are.

Another thing to consider is that oils have gotten much better since the 1150 was introduced. One of the reasons BMW recommends a thicker oil is because the oil will shear down and get thinner, A modern 10W40 or 5W40 full synthetic may actually be thicker at operating temperature than a conventional 20W50 after a couple thousand miles after the 20W50 has sheared down.
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Found this on a BMW oriented website:
"From my BMW R1100GS owners manual:

Engine Oil Viscosity (air temp. in degrees Fahrenheit):
15F thru 85F> use SAE 20W50
5F thru 85F> use SAE 15W50
5F to <85F use SAE 15W40
-5F to <70F use SAE 10W40"

Ragtoplvr correctly diagnosed the start up noise as the left side chain tensioner. I replaced it recently and start ups are much quieter.

The majority of responders suggested a 15/ 50 and so that’s what I changed it to. I picked up Mobil 1 synthetic and start ups still sound good to me. No difference in operating/ performance/ mpg between the 5/40 and 15/50 that I notice.

I appreciate all the interest and advice!