1993 Other Vauxhall Models Warning light

Hi! Suddenly last night my warning light came on and with it the battery light. No dimming lights. The alternator and plugs were changed last year and it had a service 5 months ago. In the cold weather i have to warm it up really well for 5 - 10 mins otherwise, when I put it in drive, the automatic transmission light flashes. Have I got the wrong oil in or do I need to put some more in?
Thank you
Helenx