7.62 18"bbl Varget or H4895

Depends on the gun and port pressure.
If the rig cycles well, and the bolt hold open on empty, then if it ain't causing problems and the groups are good, why fix it?
Care to share any description of the rig other than 18"?
 
When running a gas gun, try to start with load data recommendations from the folks who built the rig. They know how they balanced and tuned their system. If they didn't give you any recommendations, let me see if I can find the old article on service rifle loads for the M1A by the late Glenn Zediker and link it here. (How time can fly... seems like he hasn't been gone that long but the links have gone dead so I'll dig it out and upload it.)

Those loads were all gas gun oriented so they worked the M14/M1A, but still always worked well in AR10/SR25 patterns too. I'm sure you have seen most of this stuff, but it can't hurt.

And these are velocities that don't match your bbl length or exact bullet, but just for perspective here are service rifle loads from Hodgdon. You can get the idea of how your Varget and H4895 loads stack up.
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AR 10 308 Gas gun 16" barrel 6 lbs, 2 oz with out scope.
It likes a number of powders, 1/2" 5 shot groups can be obtained. 168ELDM at 2756 fps a max manual load for this powder, for speed. IMR 4895 is very accurate too. As are the 169, & 177 SMK.
A light fluted SS barrel, carbon fiber, titanium, and magnesium....it will often put 2 or 3 in almost the same hole at 100 yds
You can even use LC brass, no brass prep, but that may be the cause of the occasion flyer off the center of the group...or the shooter.
 

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I'm loading 42.2grs of Varget behind a 165gr bullet. Is that on the light side?

It's middle of the road. I hit pressure with Varget, Starline, and 155gr at 45.2gr with an 18" and LMOS. You will run out of case capacity with Varget and 165gr bullets. Depending on the gas length of your barrel, 4895 may be easier on your brass due to it's faster burn rate.

Any load data with Winchester brass won't be relevant for Starline brass. There's a 2gr+ capacity difference between the two kinds of brass.