Is it ok to use this tester for the 20a power outlet (not gfci)?

A Very good question, since the device name starts with GFCI. Answer is YES, because despite the name, the device is mainly a regular outlet tester, checking cross wiring, missing wiring, and reporting voltages, all worthwhile tests for non-GFCI outlet. Also works with 15amp & 30amp with adapter. Thanks for the question.

Most of the outlets in the basement where i'm living say >6.00 when i test them. are they wired wrong? is it safe to use the outlets?

It is not safe to use those outlets. If an outlet is good, the green light on the tester should illuminate, and the display should register a voltage close to 120 volts.

Is this accurate enough to measure with some confidence which is the most upstream receptacle in a circuit for choosing gfci location?

Resolution accuracy inadequate for that task. Displayed digits determines resolution. In this case, meter accuracy will not be and can not be better than +/- 1 volt because digital resolution is always +/- 1 in the least significant digit. Example: If measuring a true, standardised 120 volts, meter might display 119V (-1), 120V or 121V (+1) if calibrated/adjusted correctly. You've got the right idea, but this meter displays only 2-1/2 digits (120V). You will need a meter with 3-1/2 (120.0V) digits displayed. Worst case resolution at 120 volts will then be ~0.1V (100 mV). You need to see those millivolts differences. Get one with stated accuracy and preferably calibrated. Cheers!

ASIN	B08QW7K1JJ
Best Sellers Rank	#309,216 in Home Improvement ( See Top 100 in Home Improvement ) #457 in Multi Testers
Brand	KLEIN TOOLS
Brand Name	KLEIN TOOLS
Colour	Multi
Customer Reviews	4.8 out of 5 stars 10,763 Reviews
Global Trade Identification Number	00092644693212
Included Components	2 x 1.5V AAA Alkaline Batteries and Manual
Item Dimensions	11.5 x 2.9 x 5.1 Centimeters
Item Length	4.5 Inches
Item Type Name	Klein Tools RT250 GFCI Receptacle Tester with LCD Display, for Standard 3-Wire 120V Electrical Outlets
Item Weight	118 g
Item Width	2 Inches
Item dimensions L x W x H	11.5 x 2.9 x 5.1 Centimeters
Manufacturer	Klein Tools
Maximum Operating Voltage	120 Volts
Measurement Type	Voltage Tester
Minimum Operating Voltage	120 Volts
Model Number	RT250
Power Source	Corded Electric
Power Source Type	Corded Electric
Specification Met	CSA
Style	Receptacle Tester
Style Name	Receptacle Tester
UPC	092644693212
Unit Count	1 Count
Upper Temperature Rating	60 Degrees Celsius

Klein Tools RT250 GFCI Outlet Tester with LCD Display, Electric Voltage Tester for Standard 3-Wire 120V Electrical Receptacles,Green/Red

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