I've always used Grade 5, mostly because that was what I had available and they are plenty strong backed up with a very thick washer.  Never had a problem.

I've attached a link below that will help you chose.

Honestly Johns right ,whatever bolts we can find in the Bin possibly More important is the stand itself ,alot of stands arent rated for a fullgrown BBC Ive got close to a dozen stands some are only good to 750 

That sucks, I've done the same thing.... but it was for  a customers motor

Question:  How come no one builds a motor stand with four or five wheels like every office chair on the planet?

If it can keep fat ass secretaries from hitting the floor cuz they weren't paying attention it seems like it would be a better way to go for motors too.  I bet every guy that ever dropped a motor would immediately buy one.  Damn did I say that out loud?  I want a commission bitches!

can't spin it, but it will never drop my motor and easy to move around

You lazy ass lol 

I hear ya... I'm the same way    one hour of hard can turn into a 2 hour stretching session just to get moving again.

I've over built my shop engine lift and added adjustable legs to add ballance when needed. I once started lifting the front end of my 76 Ford
F150 4x4 off the ground when I was trying to remove 390 motor and tranny...  forgot to remove motor mount bolts first     lol.



For the price difference between grade 5 to grade 8...   for lifting!   I'd get the grade 8 bolt and washer!     Overkill is still cheaper than broken parts!
Never know when you'll find that defective grade 5 bolt made in china in your collection 
...and don't forget to use grade 80 or better chain too!  Ooooh, and some alloy grab hooks while your at it. 

-Rebel