De todas maneras manda un correo.al Parts con la duda a ver como respiran.
Hi Enric,
greetings from a cold and windy Portsmouth :-(
(es que le dije "Greetings from Barcelona" al principio del mail 
) This oil has been recommended by Mobil for both
normal and LSD diffs, we asked Mobil about every
model of MX-5 to make sure we list the correct
grade. In saying that from what you have described
you have a normal diff anyway, with a LSD if you
spin one wheel the opposite wheel should turn the
same way, if it turns the opposite way you have a
normal diff. I hope this helps you,
Regards,
Chris Davidson