Have a look at the eGuages.com website for VDO senders for their gauges. VDO was a main supplier of instruments to Volvo (along with Yazaki for some 700-cars). One of those names may be printed on the back of the cluster. A VDO oil temp sender is likely right for your gauge.

Cautionary note - I got a VDO drain plug sender many years ago from a local shop which supplied aftermarket parts for Euro cars. The guy showed me the VDO catalog where it said the sender was correct for my drain plug in the B23F. (Pretty sure it's the same drain plug in all red blocks: 3/4 x 16). It doesn't fit - it's loose, so maybe a metric M18x1.5, sorta close to the 3/4-16.

Your oil filter has that same 3/4x16 thread spec so you can check the fit of a sender on a spare filter without having to drain oil and check the pan threads. Or get a spare oil pan drain plug and drill and tap it for the 1/8-27NPT VDO sender.

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Bob: Son's XC70, my 83 244DL, 89 745 (Chev LT-1 V8), 98 S90 (recently sold) and 2010 XC60. Also '77 MGB and four old motorcycles