I wouldn't run a part synthetic oil in a Tubby to be honest.

Go for Fully synthetic next time and yes you should definately change the filter at the same time whenever the oil is changed.

Oil changs is recomended every 6k miles and should be fine for most people.

If you are hammering your car around tracks and stuff then change it more regularly.

Both oil and filter are cheap

(relatively) especially if you do it yourself which isnt as hard as you might think if your not already comfortable with doing it.

Make sure whoever sells you a new oil filter gives you the right one.

Last time i ordered online I got sent a filter for an NA which is shorter than the tubby one.

Think the tubby one is 13cm long IIRC.

Sure someone will confirm

Tom