Sorry I have no experience of your car, GoMoG.com is a fantastic resource, other than that generally gearbox and axle fill point also acts as the level..?
There is a Morgan Four owners 1936-1981 workshop Manual from Brooklands Books OWM 796, which might be handy to have, though no mention of gearbox oil level that I can see.
As for the correct grade of oil... A bit of research required though there are specialist oil suppliers for classic vehicles that might be worth a try..?
