I would do two things: look at the current oil condition. Is it dark? Smell weird (like fuel)? Then I would look at the underside of the oil cap and the inside of the valve cover. Is there varnish/thick gunk on the underside of the cap? Is there a dark coating over the valve train?

If the oil is dark, and but valve train looks good, I wouldn't worry about what oil you use.

If the cap has gunk all over it really thick, and the valve train is varnished, then I would consider using a cleaner like Auto-RX - but purely at your own discretion.

I did the 90K service myself with the service kit from Sewell. I acutally did the 60/90K at one time since my car was maintained poorly by the previous owner. It was hoses, belts, coolant change, ATF drain, oil/filter change and, diff fluid, rad cap... all that stuff was about 350 and labor was all me.

These cars are SUPER easy to work on for the most part. This was the first car I ever did bushing replacement to and the design is a little funky but rather easy to disassemble. Don't be afraid to do it yourself, start small and work your way up. Most shops are there to **** you.