Don't push it. Before about 1970 it was all guesswork. That old 1988 bag's down is probably moth poop by now.
Well, I don't know if this is a testiment to lower fill down or the quality of the bag, but...
I've got a Marmot Osprey bag I bought back in the late 80s. Semi-rectangular, 30 degree rating,600 fill down, just under 3 lbs, with a couplet sheet that allows you to unzip the bag and use it as a comforter for a two person setup.
Put some crazy use on that bag for well over a decade. Then, bought a new bag and put the Marmot up in storage stuffed in its stuff sack. This past December, after about a decade of compressed storage, I took the bag down from storage and was figuring to sell it. Thought it would still do great as a sleep system for a couple.
Went to the laundromat, washed it and dried it with a few tennis balls thrown in to help break up the down clumps. Got home and finished unclumping and shifting the down. Finished up, zipped it up and layed it out on the bed. I swear that bag looks as good as new and hasn't lost a mm of loft.
Took the bag with me and used it for a trek to the Mojave National Preserve the first weekend of January, temps down to freezing. Slept ever so comfy with the lightest of base layers.
Now, I won't take the Marmot on the trail; I've got a much lighter 20oz 750 fill bag that does everything I need (and a lesser 20oz'r that works beautifully as an overbag). However, though I was planning on ditching the Marmot, thinking its best nights were behind it, that baby is staying with me and is at least going to be the new bag for base camp when I'm out on mountain biking getaways.
No moth poop here!
Still warm and lofty after all these years!