You can but it will be messy and you will loose some of it. Most of the oil is in teh pan, but there is still lots of oil in the lines and filter. Oil isnt that expensive, just change it with the filter.
I run Amsoil as well, but I can't seem to bring myself to leave it in that long. I look forward to driving this car when its got over 200k on it though and would hate to skimp on maintenance intervals just to save a few bucks.
You can certainly just change the filter. You'll lose about 1 qt of oil. This is actually a good way of replenishing the oil when doing longer change intervals. I don't really think that an additional filter change is necessary in your case, especially if you're using the 4.3L filter, but if it makes you feel more comfortable, then by all means go ahead. It'll be the same mess as any other filter change.
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