My local dealer uses Pennzoil 15W-45.  Go try to find that in your auto 
parts store.  Of course, they sell it at a premium price and they are the 
only available source once it's in the crankcase.  I complained and told 
them I was using 10W-30; they said the best alternate was 20W-40 or 20W-50.  
I doubt any of these grades would really cause any problem if they are 
changed frequently and you don't live in Fairbanks or the Sahara.

C. Vaughan
'84 GL
Castrol 10W-40