jeez I never realised the pressure switches were so low, by the time they light up you've had it anyway at 0.3bar ?
how many wire circuit is it ? could it be a plug or loom fault ?
a gauge actual value would be quickest and set your mind at rest also, if ok your onto elec fault.
total guess id have to say 2 or 3 bar cold engine whatever oil, some 5w oil has viscosity close to 10w anyway as they are classed in bands
heres a link to selinia oils
and they list the 100c but not the 40c and depending which 10w40 it is the 5w or 10w are probably pretty close at 40c like 85cst vs 80cst but the 100c is quite a bit thinner 40 to 30 maybe eg 14cst vs 10cst(made up value from gulf racing specs)
surely the only reason to use lower high temp viscosity oil is mpg at the cost of protection. which is why manufacturers have reduced them ? to gain mpg figures to sell cars.
edit, cant understand why triggering fault on cold engine and not warm? pressure should be higher cold , any chance issue is with anti drain valve in oil filter ? could be draining out and seeing no pressure till filters refilled, how fast does the light go out started on cold engine, seconds or minutes.