Hi Paul.
I had a "grain" problem recently using oils and turpentine mix,and I think it was down to not having mixed the wash enough.
Ive kind of given up on oil washes at the moment,they never seemed to flow right for me,and Im now moving more towards heavily diluted acrylics which sofar are working better.If you are "washing" over a Klear/Future base you can wipe the water based mix off and not leave the oily smear I used to get with oils.
This is of course purely a personal preference,and lots of people use oil washes on an everyday basis and have no trouble at all.Washes are not my strongest point by a long way and Im continually experimenting until I find what works best for me.
Hope this rambling helps.
Nige