Hello Robin,
It seems Christie has not been on here for about a week so we don't know how he is getting on with this. Shame.
Perhaps the oil way you are talking about is the oil feed from the pressure side of the oil pump, which pumps oil up to the overhead camshaft ( with the oil returning to the sump via. numerous large cast in holes which allows the oil to get into the sump in the first place when the oil is changed/topped up). If this oil way was blocked it would prevent oil getting to the top of the engine, but not necessarily cause the oil oil pressure light to come on.
I just wonder with your Astra if it suffered from a previous glorious lack of oil changing, and/or coolant entering the oil via. a head gasket issue - causing the oil to emulsify and eventually clog up the oil way.
Mixing grades and brands of oil being a big no, no? This should not be an issue, or a cause of oil sludging in its own right. Although you should always use the recommended viscosity and specification as the first choice, with high mileage or worn engines it can be worth reconsidering alternative near-grades. Mixing same specification brands leading to blocked oil ways? Never yet come across this in over 55 years of motoring.
Besides do the usual frequent fluids checks and tyre condition and pressure checks, probably the most important part of maintenance is to treat the vehicle to good quality oil and filter changes - it does reap rewards. So called long- life schedules? OK they have their place but for old fashioned me it's a 10k mile/ 1year job for me.
Kind regards,
Gareth.