Over the last few years I have had this irritating problem with my Beta 50.
I have changed the sender and changed the gauge a couple of times and was even convinced that the engine was at too slower tickover.. However, reading through the Beta service manual for the umpteenth time I have been made aware that there is an adjustment that can be done.
So, I took out the gauge and there seemed to be a brass bit with a hole in it. Having had the problem for so long and that it was really starting to wind me up, I had to have a go!
I managed to get a very small screwdriver to grip the side of this brass thingy. I gave it a turn, it moved, I turned it again to something like halfway around, reinstalled the gauge and during the last two weeks of cruising the oil alarm has not come on again.
Tony will probably know what the brass thingy does, but it seems to have stopped my problem at the moment.
Now if only i could cure the engine not starting when the key is turned, I'd be a happy man!