I'm planning on converting my 89' 740T A/C this evening using the basic kit from Volvo. I've heard conflicting advice concerning draining all the old R12 oil from the compressor or just blow in the new oil provided with the kit. Some say it is compatible some say it's not. Same goes for the compressor seals, I heard you should change them some say not. Please advise.

The kit from Volvo is the right one.

Drain ALL the oil from the system including the compressor. The Diesel Kiki compressor is fine; don't bother with shaft seals (but DO change the compressor inlet/outlet O-rings which might need to be purchased separately from the kit for some dumb reason). The Sanden compressors (some) leak at the shaft seal when converted.

After draining all the oil, change the parts and put in the new ester oil, then evacuate to a deep vacuum, and charge with 80% of the R-12 amount in the system. Volvo publishes the correct figure.
--
1992 940 wagon, 76k
as well as others.

I have had two Volvo 740(s) switched to R134...you can just evacuate the oil freon and add the new refrigerant and oil with no problems.

Yes you have to remove the compressor to remove all the oil. The oil for r12 is different than 134a and it is incompatible. Do you have a Diesel kiki compressor? Don't do it unless you need a new compressor. Any good shop will discourage you from changing unless you are getting a new compressor. Check the FAQ.
--
92 244 lowered, Bilsteins, IPD sways, Unitek cam/header, TSW Revo 18x8's - 94 965 MONEY PIT! - 76 Datsun 280Z lowered, modified head, cam, headers, intake

Don't understand what you mean by don't do it. Please explain.